[central_va]

70 years later and it is a mystery.....

[fso301]

70 years later and it is a mystery.....

Reising M-50 may be the match.

The Reising submachine gun was designed by American Eugene Reising and patented in 1940. Production of the new submachine gun commenced in 1941 at Harrington & Richardson (H&R) arms factory. In 1942, US Marine Corps signed first contract for delivery of Reising M50 submachine guns, and several tents of thousands of Reising M50 SMG's were delivered to USMC during the war.

Read more at:

http://world.guns.ru/smg/usa/reising-m50-m55-e.html

[Ecliptic]

Looks like a sawed off shotgun to me.