What you are probably seeing is an M1928 Thompson. The M1921 was tested by the U.S. Army and Marine Corps in the 1920s, but they showed little interest in it, possibly due to its high cost, (~$200) relatively complicated and refined construction, and the services' disinterest in submachine guns and other "new" weapons during that period. The changes between the M1921 and M1928 were quite small, with a rougher blued finish, a heavier actuator to slow down the rate of fire slightly (from ~800 to ~600 rounds per minute) and a Cutts compensator added to the barrel to decrease muzzle climb during sustained fire. Remanufactured guns had the "1" of their "M1921" marking overstamped with an "8." The M1928 still had the vertical ("handle" or "handgrip") foregrip of the M1921, while the newer, simpler M1928A1 did not, having a horizontal one that mostly covered the barrel. The M1928 and M1928A1 were adopted for military service in 1938, but generally replaced by 1942-43 with the "wartime" M1 and M1A1. The M1 and M1A1 were incapable of accepting drum magazines due to their simplified design. The Thompson was replaced on paper in January 1943 by the M3 "Grease Gun" as the Army's primary submachine gun, but M3 production was quite slow and it never fully replaced the Thompson in frontline service before the end of the war.

Source:

Tommy Gun: How General Thompson's Submachine Gun Wrote History, by Bill Yenne

Basically early war like the other poster said. I believe other government Thompsons were also collected up for the war effort, as while its just family lore, grandfather claimed his M1928 he carried at Guadalcanal was marked as property of the US Postal service.