Standard issue for Army aviators were 4 inch standard (pencil) barrel Model 10s. As late as 1984 when I got out we still had a wide representation of them. Not exactly Viet Nam, but the same TO&E.
All the way from a few post WWII transitions and 5 screw no models to then current production. As I recall, the most common being 10-4 and 10-5, most did have diamond Magnas though. I am curious to know what became of these, most were fired about 50 rounds or less per year, cleaned and carried alot.
I think I still have an impression of one in a M4(?) holster in the ribs under my left arm. When they came out of the arms room, they stayed on your body 24/7.