Click on images to enlarge or to see full sized drawing.

The bottom plate that all of are used to seeing with the eight rivet flange to attach the side plates to the bottom plate and the integral T&E bracket did not always exist.  How it came into being is a long an interesting story.  

First a little background. 

In 1925 the Ordnance Department changed  model designations by dropping the “Model of” and substituting the upper case letter “M“.   In this way the Model of 1917 became M1917.  For the sake of clarity this article will refer to all weapons by the post 1925 designation.  

Sometime in the middle 1930’s Ordnance stopped using the year in the identification of weapons  i.e. it’s the “ U. S. Rifle, Caliber .30 M1” the famous Garand rifle, not U.S. Rifle Caliber .30, M1936 it’s year of adoption.  However, Ordnance continued to use the original identification on existing weapons such as the M1917 BMG to prevent having to change all previous documentation where the weapon was identified under the year of adoption system. 

From prior to WWI  until  June 1, 1931 all M1917 drawings were numbered in the Class and Division naming convention.   Automatic weapons were in Class 51 and the M1917 was numbered Division 10.  Drawings were  numbered sequentially starting with 1 which was a drawing showing views of the entire weapon..  Often in the Class and Division system of identifying drawings, more than one part was shown on a drawing sheet so a letter was added to the sheet number to denote which detail on that sheet was the part in question.   

The bottom plate appeared on sheet 10 detail C so the drawing number was 51-10-10C  The  term “piecemark” is the pre late 1943 term that Ordnance used to indicate how the part was numbered  and in some cases how it was marked.  The original M1917 bottom plate was assigned a piecemark of 10C

While every part had a piecemark not every part had that identification imprinted on it, some were just too small and, possibly, as a matter of secrecy during WWI, none of John Moses Browning’s patents for the M1917 were filed until after the armistice on November 11, 1918, as after filing, the patent drawings become public record.   Original WWI parts were  usually marked as to the manufacturer, but not usually with a piecemark to identify the part.

Original  production M1917 machine guns had a bottom plate that slipped into dovetail grooves machined into the side plates, and was both peened into place and held  in position by the back plate. Farther on in this discussion you will find out the significance of peening of the side plates.

In 1922 Ordnance implemented a change in the way that drawings were identified.  Standard  drawing sizes were mandated and identified by the letters A thru E with A being the smallest and E being the largest.  All of Ordnance’s drawings were redrawn on the new size medium and renumbered starting with 1 for each letter size.  In addition, only one part was supposed to be shown on each drawing. 

During the interwar period which included the Great Depression, drawings were slow to be converted to the letter size format due to a lack of funds.  In the case of the  M1917 bottom plate along with other parts, it wasn’t converted until June 1, 1931, when it became C8464.

When the M1917 was introduced into combat  late in WWI, reports of failures of the bottom plate dovetails, side plate grooves, broken cam blocks, cam block screws, and bulging side plates began to filter back from the front.  WWI ended before  anything could be done about this problem.  Post war tests concluded that a reinforcing stirrup  could be retrofitted to existing weapons to solve the immediate problem.   This stirrup was about 2 ˝ inches long with an approximately 1” flange on each side and a hole in the bottom to accommodate cam block screw. It was riveted to the side plate with 2 rivets on each side.

Drawing 51-10-17 the reinforcing stirrup and the attaching rivet. Note that while the portion of the drawing shown is detail A on sheet 17 the piece marks are 17G for the stirrup and 17H2 for the rivet.   Courtesy Jodie Creen Wesemann, RIA Museum

Much finger pointing ensued over the failure of the M1917 to perform as advertised.  Accusations of side plates fabricated from the wrong type of steel, poor workmanship, poor inspection of the weapons, and poor Ordnance oversight of productions swirled around.  The truth of the matter is that the failures can be attributed to inadequate design which would have become evident had there been time to adequately service test the weapon before introducing it into combat.  Even John Moses Browning didn’t bat 1000.

Starting in 1920 about 25,000 reinforcing stirrups were manufactured and installation of the stirrups was begun at the Springfield Armory where about 10,000 weapons were retrofitted thru 1926. Other ordnance facilities installed the stirrups both on a repair and return basis and inspection and refurbishing prior to storage operations.

Drawing B131261, the reinforcing stirrup.  Courtesy Jodie Creen Wesemann, RIA Museum

All quiet on the bottom plate front, sort of.

In 1922 an Ordnance officer stationed at Springfield Armory, one Capt. Walter T. Gorton, designed and patented a new style bottom plate which included an 8 hole riveting flange and an integral  T&E  bracket that eventually became the bottom plate we are all familiar with.  Since Capt. Gorton was an Army officer, his patent was assigned, without compensation, to the United States Government. 

Meanwhile, the Cavalry arm of the Army had been searching high and low for some sort of light weight automatic weapon for pack mounting. They briefly thought that that a modified BAR, the very short lived M1922, might be just the ticket, but later thought better of it.

Since nothing seemed to be forthcoming from Ordnance, in 1930 they took the bull by the horns, acquired a M1919 tank gun and modified it for Cavalry use and submitted it to Ordnance as a prototype  eventually this weapon became the M1919A2 air cooled BMG.

Not to be outdone the Infantry, in 1931, requested the Ordnance Department to provide 72 Browning Tank Machine Guns for testing and development of field doctrine.   Both the Cavalry and Infantry weapons had the 18” barrel with the slotted jacket.  By 1935 the Infantry had concluded that a longer barrel was required and the now familiar 24” barrel was chosen. 

A parallel development was occurring at the same time as the Infantry and Cavalry were testing their new air cooled weapons. Ordnance resurrected Capt. Gorton’s bottom plate design, tuned it up a bit, and produced a new bottom plate which looks much like the plate we know today.

For some inexplicable reason Ordnance decided to number this drawing of Capt. Gorton’s improved bottom plate C8464, the same number as the dovetail style M1917 bottom plate. 

Drawing C8464 dovetail bottom plate WITHOUT flanges. 
Courtesy of Jodie Creen Wesemann, RIA Museum.

Drawing C8464 bottom plate WITH flanges.
 Courtesy of Jodie Creen Wesemann, RIA Museum.

We are not in possession of all of the drawings, however, it appears that this new and improved bottom plate was introduced into the system in the fall of 1936. 

We are in possession of  drawing C8464 Revision 5 (1-15-37) which shows the familiar flanged bottom plate.  Revision 5 also shows that this design pertains only to “BMG 1919A4”

Time marches on, and by February 1, 1938 the C8464 drawing had become drawing D35392, probably because of the ensuing confusion over two different parts that were not interchangeable, having the same piecemark and drawing number.  There are only very minor dimension modifications between the two bottom plates drawings, and the “Drawing Pertains To” block on drawing D35392 shows 30BMG-M17(WC), 30 BMG-1919A4 FIXD, 30BMG-1919A4 FLEX, and drawing E1301 CASING, W/WATER JACKET ASSEMBLY, which is most likely the M1917A1 full sized right side view drawing.   E sized drawings were 40’’X necessary length, and to show a M1917A1 full size, you need a lot of paper.  M17 is most likely a non standard M1917 or M1917A1 abbreviation.

Drawing D35392 Revision 16 dated 8-15-45 depicts the final WWII version of the bottom plate. This drawing shows the piecemark (part number) D35392-10. The black rectangular markings surrounding the drawing appear to be sprocket holes in the 35mm microfilm used for holding the film in position. Most Ordnance facilities used 35mm microfilm as a method of archiving drawings although some documents were preserved on 70mm film. Courtesy of Jodie Creen Wesemann, RIA Museum

Right about now RIA became the Ordnance facility responsible for Cal .30 BMG production, replacing the Springfield Armory, which was up to it’s necks in M1 Garand production. 

In 1938 RIA began to convert M1917’s  and M1919A2’s to M1919A4’s, this fits nicely with the 2-1-38 bottom plate drawing number change from C8464 to D35392.   More than likely, some  stock of flanged C8464 bottom plates had accumulated.

Since, as of yet, we have seen few C8464 imprinted bottom plates,  it is not clear how many Ordnance facilities actually manufactured this bottom plate.

The  M1917 rebuilding/modernization/modification/conversion, these terms seem to have been used interchangeably,  came on November 16, 1939  and specifically called out, along with a laundry list of other changes, the use of the D35392 bottom plate in the rebuild process.  Also included in this approval was notification that funds were available to convert 7,800 weapons.

The M1917’s converted to M1917A1’s  and some of the M1919A2’s and M1919A4’s that were previously converted  from M1917’s  required a slightly altered  (shallower) milling  depth of the inside of the bottom plate. This resulted from the fact that the original, unaltered M1917’s had the  bottom of the side plates  peened upward to hold the original dovetailed bottom plate in position.  This peening and subsequent milling of the bottom plates reduced the height of the side plate by .020 to .040 inches and when the standard D35392 bottom plate was installed at rebuild, it changed the dimensional relationship of the internal parts to the casing potentially causing functioning issues.

Shown above is the underside of an original New England Westinghouse M1917 machine gun converted to a M1917A1 at the Raritan Arsenal.  Note D35392MP-RIA imprint and the original cam block screw staking.  Note the four bosses to the left of the T&E bracket which are present on all flanged bottom plates, they are designed to accommodate the pack frame for horse transport. Private NFA collection photo.

This created an interchangeability problem, as the modified weapons would not accept a standard C8464 flanged bottom plate, or new manufacture D35392 bottom plates.

The fix for this was to mark either the C8464 or the D35392 plates modified to work on the M1917 or M1919 rebuilds  with the drawing number, sometimes with a revision suffix number,  and the letters “MP” signifying, we believe, “Modified Part” along with the identity of the arsenal which produced the modified  plate RIA is the only one seen so far.

Fig 2 
This D35392MP-RIA bottom plate is from what we believe to be a fairly early M1917 to 1919A4 RIA conversion.  It is now a semi-auto 1919A4 built up from a ”parts kit” that contained a barrel extension, bolt, and trigger with Westinghouse markings.  The left side plate has  two vacant rivet holes left over from the stirrup removal during the conversion. The parts kit also had an early style back plate with the aluminum stocks (grip panels).  Photo courtesy of “CaptMax” 1919a4 forum.

The weapons with the non-standard bottom plates worked just fine, and now there was a  simple way to tell new production weapons from rebuilds that required non-standard bottom plates.  Just turn the weapon over and look at the bottom plate to see if it was marked with a MP suffix.

This was not a good situation because it created another problem, confusion between MP and standard plates since the same drawing number, either C8464 or D35392 was on both. 

To remedy this new problem, RIA prepared a drawing showing ONLY the difference in the inside milling of the plates used on modified/converted weapons and a note to see drawing D35392 for all other dimensions.  RIA/Ordnance numbered this drawing  D37887 dated July 12, 1940 and began marking the modified plates D37887-RIA.

D37887-RIA bottom plate used to convert M1917 and M1919A2 to M1917A1 and M1919A4 weapons. Photo courtesy of Rollin Lofdahl

It never fails, fix one thing and something else breaks, now there were three differently identified plates in the field all doing the same thing.

Apparently, this was deemed a good situation, because nothing further regarding bottom plate numbering occurred.

The very rare C8464 MP plate used to convert M1917’s to M1917A1’s or M1919A4’s. Some MP plates were also used to convert the few extant M1919A2’s  and M1919 tank guns to M1919A4’s. This is the  “Holy Grail” of bottom plates and the only example of a C8464 MP plate that we have observed. Note the stamping of “-7MP” which appears to be done with individual letter stamps.  This plate dates, most likely from the late 1930’s.                    
Photo courtesy of “jmb1955-1926” of the 1919a4 forum
As of now we have discovered only one C8464-7 MP RIA bottom plate.  It is not attached to a side plate. Additionally, we have identified one M1917 to M1917A1 New England Westinghouse conversion in original configuration, right down to the original cam block screw staking, rebuilt at the Raritan Arsenal, at a time not known, and one 1917 to 1919A4, a semi-auto conversion, that came as a kit with a left side plate attached to the bottom plate.  These two weapons have D-35392-MP RIA bottom plates.
One picture is worth a thousand words.  This one is probably worth more than that. The bottom plate on the left is the C8464 RIA -7MP  shown in the above  photo.  This clearly shows the shallower depth of the side plate grooves necessitated by the shorter peened/milled M1917 side plates. The right photo shows a standard D35392 bottom plate

Both C8464 MP photos courtesy of, and a special thanks to “jmb1855-1926” from the forum.

This photo shows a nonstandard bottom plate apparently modified to salvage an otherwise junk weapon. Photo courtesy of Matt Danker

We have also encountered several RIA D37887 plates unattached to any other part of a weapon, and several built up into weapons or still attached to a left side plate, as  an incomplete weapon.  All D37887 encountered so far are RIA produced.

M1917’s were converted to 1917A1 configuration not only at RIA (Rock Island Arsenal, Rock, Island, IL) but also at AA (Augusta Arsenal, Augusta, GA,)   BA (Benicia Arsenal,  Benicia, CA),  MR (Mt. Rainier Ordnance Depot, Tacoma, WA),  RA ( Raritan Arsenal,  Raritan, NJ),  RRA (Red River Arsenal, Texarkana, TX),  and SA  (Springfield Armory,  Springfield, MA) 

It appears that all, or nearly all, of the M1917 and M1919A2 to M1919A4 conversions, were performed at RIA.

Apparently, units equipped with M1917’s shipped their weapons to the closest facility for either rebuild and return, or more likely they received rebuilt weapons, and used the crates to return  the ones needing modification to the supplying facility.  The M1917 to M1917A1 rebuild program extended all the way until the end of WWII.   

SNL A6, List of All Parts for Browning M1919A4 Fixed and Flexible; M1919A5 fixed; and M1919A6, Flexible; and Ground Mounts dated 6 September, 1943 carries a note on page 21  Note 1. Plate, bottom D37887 should be requested for guns that have been modified.  These guns may be identified by the marking C8464-MP, D35393-MP or D37887 which appear on the underside of the bottom plate.” 

This note does not appear on the same publication dated May 1941 or ORD 9, the new designation for the “List of All Parts” document, dated January 1944.

This tells us that the MP plates were no longer being produced, having been replaced by the D37887 plate, however there were probably some of MP plates remaining in the supply pipeline.

Identifying the bottom plate can be a little tricky.   WWII era bottom plates were made  from rough forgings machined to final dimensions.  While post-war bottom plates could be an alternate design cast from an alloy developed by  the Saginaw Malleable Iron Division of Saginaw Steering Gear.  Saginaw’s Trademarked alloy, ArmaSteel, was used to make several of the M1919s parts besides the bottom plate. These cast parts included one piece booster/bearings, trunnion blocks, top plates, latches, and a one piece back plate with an integral pistol grip.

The use of ArmaSteel, a perlitic iron alloy, speeded production by reducing the time needed to finish the parts, and cut steel requirements by reducing scrap inherent in conventional forging/machining production methods. 

Obviously it is very easy to tell a C8464 dovetail plate from the C8464 plate with riveting flanges and integral T&E.

Some of the original dovetail bottom plates left from WWI may be piece marked 10C, most will not, most should have the manufacturers symbol :

R in a triangle, Remington Arms Co, Bridgeport, CN,

W in a circle New England Westinghouse, East Springfield, MA,

C in a box, Colt’s Patent Firearms  Mfg. Co. Hartford, CN

The first drawing of the dovetail C8464 plate dated June 1, 1931 did not indicate placing a piece park on the part.  The next available drawing C8464 Revision 1 dated 6-27-34 indicated that the parts produced from this drawing should be imprinted  in 1/16”  characters “C8464-1”  between the cam block screw hole and the 4 holes for mounting  the T&E bracket. 

The last drawing that we have of the  dovetail C8464 plate,  Revision 3, dated 3-15-36 still shows the “C8464-1” piece mark imprint required.  According to this information, the only C8464  piece marked dovetail bottom plates will be marked “C8464-1”.

As mentioned earlier, about 9-25-1936  the design of the bottom plate changed to the one familiar to us today. However, for no good reason that we can determine from this distant point in time,  Ordnance kept the original drawing number, C8464, for a totally redesigned, and non-interchangeable part. 

The drawing C8464 (flanged) revision 1, dated 9-25-36 did not require a piecemark.  The last available C8464 (flanged)  drawing shows Revision 6, dated 1-15-37, still not requiring a piece mark.  This does not mean that the parts were NOT piecemark imprinted it means that they were not REQUIRED to be imprinted.

Enter the D35392  bottom plate drawing dated February 1, 1938, its virtually the same as C8464 (flanged), Revision 6, it does not require placing a piece mark either, but it calls out the piecemark D35392.  The next available drawing is D35392, Revision 1, dated 3-10-39 still calling out the same piece mark but  not requiring imprinting.

Next comes drawing D35392 Revision 2, dated 3-19-40 which changes the piece mark to D35392-2 still no requirement to imprint the part, and the only observable changes in the part was a change in tolerance on the thickness of the riveting flange with the Revision 2 increasing the tolerances to +.003 from +.001 on the Revision 1 drawing.  This also changed a couple of other tolerances up to +.006.

The first mention of a piece mark imprinting requirement is on drawing

D35392, Revision 7 dated  3-9-42 shows a piece mark requirement by Revision 6, but the drawing is torn and the piece mark area is missing  We don’t have the Revision 6 drawing, but we have the date for Revision 6 it’s 1-19-42  and the required piece mark was, by backtracking the revision numbers, D35392-2.  This is one of the very few occurrences that we have observed where the piecemark suffix and the piecemark imprinting requirement have different revision numbers.

Now we have a piece mark  D35392-2, AND an imprinting requirement Revision 6, with a date certain, 1-19-42.

This bottom plate is a D35392-2 SG many Saginaw Steering Gear produced bottom plates are unmarked, even though Ordnance required markings.  Sometimes Saginaw bottom plates can be identified by the presence of an Ordnance “Flaming Bomb” on the right riveting flange near the fourth rivet hole.  This particular marking seems to be 1/16 inch rather than the mandated 1/8 inch characters.  Because of the revision suffix number 2 the production of this plate can be roughly dated between 1-19-42 and 3-17-43. 
Photo courtesy of Michael S. Gorham

The next change in the piece mark is drawing D35392, Revision 10, dated 3-17-43. Revision 10 made a slight (1/64”) change in the tolerance of the radius cut on the inside of the riveting flange, and added a note about reaming the T&E holes “.4400-0015  REAM AT ASSEMBLY”  The new piece mark was D35392-10.

D35392-10 is, as far as we can tell, the final WWII version of the bottom plate.

We have drawing  6535392 Revision 17, which was the revision that renumbered the  bottom plate drawing yet again, and re-drew the drawing cleaning up all the revisions previously made, dated 5-10-48.  6535392, a D size drawing,  also changed the piecemark, now called a part number, to 6535392.  This drawing also carries the imprinting requirement, and the notation “Was D35392”.  In addition, we have observed one bottom plate marked 6535392-22 RIA this is believed to be Korean war vintage production.  We, as of this writing, do not have a 6535392 drawing showing what Revision 22 actually accomplished.

 Fig 5 6535392-22  standard (Revision 22) bottom plate.  In May of 1945 drawing 6535392 Revision 17 replaced drawing D35392 Revision 16.   This was the result of finally completing the 7 digit  drawing number conversion that started in 1943. The conversion went so far as to continue the revision numbers from 17 onward. This plate is, most likely,  Korean War vintage replacement part, as it shows little wear or evidence of ever being assembled to the side plates. Photo courtesy of Rollin Lofdahl

If you kept the everything in the last several paragraph straight, you are doing very well.  We have all of the drawings at hand, and spent hours trying to get this right. Keep in mind, that if it’s a 6535392, D35392 OR a C8464 with a flange, no matter what the number suffix it is interchangeable, PROVIDING that it’s NOT marked with a “MP” suffix also.

There is no evidence as of yet that any of the  D35392-MP plates were manufactured any where else than RIA, or that any of these plates have revision suffix numbers. We suspect that RIA being in charge of the Cal. .30 BMG production produced all of the MP plates, but this just a supposition. We also suspect that RIA produced all of the D37887 conversion bottom plates, another supposition. What we don’t know is where the heck all, the C8464-MP plate have gone off to. An educated guess would be, there just weren’t all that many of them, and just like they used to say on the X-Files “They’re out there” we just haven’t encountered very many of them yet.

In addition, when piece marks started to be required to be imprinted on the parts, the manufacturer was required to imprint a code showing who made it. The known WWII makers of .30 caliber BMG’s, in addition to the previously noted WWI manufacturers are:

RIA (Rock Island Arsenal, Rock, Island, IL)

BA, BAC has been reported (Buffalo Arms Corp, Buffalo, NY)

SG (Saginaw Steering Gear Div. General Motors Corp. Saginaw, MI. Some parts may be found imprinted “S.G.“ these parts are believed to have been  produced by Saginaw‘s Grand Rapids, MI plant)

BCI, BC has been reported (Border Cities Industries, Windsor, ON)

Border Cities  Industries was a General Motors, Canada subsidiary which manufactured BMG‘s  for  British Commonwealth countries.

Sometimes parts were produced by subcontractors, and those parts were marked with letters or numbers, codes to identify what subcontractor or even what subcontractor’s, or the manufacturer’s own,  production line fabricated the part. That is a subject for another day, and most likely,  looking back nearly 70 years, we will never know for certain what  all of the manufacturer’s marks meant. Saginaw, we don’t know why they did it,  had a habit of stamping the Ordnance “Flaming Bomb” on the right riveting flange near the 4th rivet hole.

Fig 3 D35392-2 bottom plate manufactured by  BA (Buffalo Arms Co.).

The D35392 Revision 6 drawing required placement of this piecemark on the bottom of the plate.  It is unknown why Buffalo Arms placed the marking in this location. Photo courtesy of Tim Gaine.

Buffalo Arms, not to be outdone, took to stamping the required drawing number/part number/piece mark on the riveting flange, again, we don’t know why, when the drawings clearly called out imprinting the bottom of the bottom plate. Nearly every day something else comes up that defies logic and muddies the waters, but that’s what makes this stuff interesting.

To sum all this up, there are various, or no markings on the some of the original dovetail bottom plates it could be marked 10C, C8464-1 or not at all.  Some  flanged bottom plates are from drawing C8464 and have MP suffixes, some  flanged C8464 plates have no markings at all.    

Some D35392 plates are not imprinted with piecemarks at all and, as a practical matter,  are all but indistinguishable from non-marked C8464 flanged bottom plates.  Some are marked as “Modified Parts” (the MP suffix plates) Some have the drawing number D35392, but no number suffix, and some either a -2 or -10 suffix. Some plates are marked 6535392 with or without suffix. If you have a D37887 bottom plate, well, you pretty much know what it is, and who made it but not a very good idea when it was made, since the M1917 to M1917A1 conversions were still going while we were celebrating  VJ Day, August 15, 1945.

By re-reading this, and comparing your plate to the pictures and text you should  get an idea of about when your bottom plate was manufactured, and maybe by whom. If you think this has been a “hard read”, you should try writing it and trying to keep all the Revision numbers and drawing numbers straight. In this we think we may have had some small success, but, no doubt, something slipped through one of the many cracks.

In any event, we hope that the reader has found this material helpful.

What success, if any, we have had in trying to shed some light on all of this, is due in no small part to the following: 

                                                                CREDITS AND ACKNOWLEDGMENT
Jodie Creen Wesemann,  Rock Island Arsenal Museum, Rock Island, IL contributed much to this effort, without her gracious assistance, we would still be struggling to grasp any of this.

The Browning Machinegun Vol. 1 Dolf  L. Goldsmith, Collector Grade Publications. INC

Hard Rain, History of the Browning Machinegun,  Frank Iannamico, Moose Lake

Publishing LLC

The aficionado of the Browning Machinegun would be well served by purchasing these books for study and reference.

To all of the members of the forum who examined their bottom plates and contributed to this discussion with comment and pictures, a sincere Thank You.

Individual photos have credits cited per the providers request.

Last but not least, my friend Rollin Lofdahl for all his help, encouragement and being an all around good guy .