Originally Posted by
Templar
The pre war Thompsons, the M1921, M1928, and M1928A1 were the models that allowed the use of both drums and stick mags, had the cocking handle on top, and used the more complicated "Blish Lock" operating system.
Once we got into the war, they attempted to cut some corners in production. In 1942 the M1 Thompson submachine gun was type classified and adopted. The Blish lock was done away with and the weapon was now straight blowback, drum mags were no longer useable, the finned barrel, Cutts compensator and Lyman adjustable sight was done away with. The cocking handle was moved to the side on the M1.
The M1A1 was even more simplified with the firing pin machined into the bolt face rather than be a separate unit.