37mm mk iii a2 brass shell casing dated 1940

[Aragorn243]

Nice, nice, nice...I have the longer projectile, M51B. My casing is the longer 37mm M16. I wonder if yours would be the type used in the P39 Aircobra?

Interesting. Following your lead and looking at the projectile rather than the shell, this is what I found:

Practice shell, 37 mm, M55A1 INERT M55 A1 practice round for the 37mm M4 auto-gun, used by the US and USSR military during WW II.

The steel case is marked with MARK III A2 and the complete round is dated 1941.

M4 auto cannon was used on Bell P-39 Aircobra and P-63 Kingcobra used by US and USSR Air forces and by US Navy PT boats

I have one of the longer versions also, they were used in the Stuart light tanks among other things.