When I got to my first assignment in 1987 the aircrew issue gun for concealment was the S&W M15 2" square butt, just like yours. That's what I drew from the armory to fly in the Panama invasion in 1989 and again for Desert Shield in 1990 and then Desert Storm in 1991. The USAF didn't go to the Beretta M9 for C-130 aircrews until a while after that. Great find and I'd love to have one myself for sentimental value.