The M-31 (Or KP-31) Suomi (Finland) Submachine gun was one of the weapons designed by Aimo Lahti.
Adopted by the Finnish army in 1931, was manufactured until 1944. Received widespread use in the Winter War, where its success would influence Soviet designs (the russian PPSH-41 mag is a direct copy of this).
Remained in use until the 1990's.
This Mag is in great condition, with Finnish markings.