The differences between aircraft and ground MGs are extensive, owing mostly to the lightened components. The differences between the Modèle 1932 and the Modèle 1938 aircraft guns are less dramatic, but involve most of the components. The Modèle 1938 was a chance to improve on the earlier Modèle 1932, when springs, side plates, sears etc. were strengthened. The vertical buffer became horizontal and a second set of mounting points was added to the bottom plate. Most of the parts shown below are from the Modèle 1932 and Modèle 1938, with a few Israeli contract FN30 parts. It is by no means exhaustive, but here is a representative group that may give you some idea of distinctions between the variants.
Bottom plates.
Aircraft gun back plates, 8mm.
Aircraft bolts have significant lightening cuts. These are both from 8mm MGs.
Note the slot for the larger sear in the Modèle 38.
The Portuguese crest was stamped on the top plates of their 8mm aircraft guns.
Firing pins, above. The later Modèle 38
has grooves around the front collar.
Belt feed slide showing the addition of a second spring in the Modèle 38.
Driving springs.
The Modèle 1938 left hand side plates show the addition of raised areas to add strength to the lightened aircraft receiver plates. The rear boss has been lengthened forward to strengthen the area around the back plate
The full ring was replaced by a small lug which prevented the charging handle from rotating when the gun was being cocked.
Most of the aircraft guns had wooden inserts. I'm not certain why this was done, as many, if not most of the inserts broke up from the recoiling of the high speed weapons.
Sight base cross-pins.
All three weapons have the short sight base.
Three views of sears.
The signature single handled back plate. This was used on most of the models. This one is from an Israeli contract ground gun, 7.62 NATO.
The FN30 has a top plate with longer sight base rails.
The bolt for the FN series ground heavy barreled MGs.
The Modèle 1952 booster.
