M1 Carbine (rifle) converted to M2 full auto with selector switch. Winchester. Chromed (?) frame. Filler behind receiver at stock. Marked M1 above chamber. At rear behind simple flip sight, marked Winchester 1133659. Initially, the M1 carbine was intended to have a selective-fire capability, but the decision was made to put the M1 into production without this feature. Fully automatic capability was incorporated into the design of the M2 (an improved, selective-fire version of the M1), introduced in 1944. The M2 had a revised wood stock and featured the late M1 improvements to rear sight, a bayonet lug, and other minor changes. Although some carbines were marked at the factory as M2, the only significant difference between an M1 and M2 carbine is the fire control group. The military issued field conversion kits (T17 and T18) to convert an M1 to an M2. Legally a carbine marked M2 is always a machine gun for national firearms registry purposes. Other changes developed for the M2 were a 30-round magazine with three catch nibs (as opposed to two on the fifteen-round magazine); and a magazine catch with a third retaining surface. These M2 parts including the heavier M2 stock were standardized for arsenal rebuild of M1 and M1A1 carbines.