The M1A1 were versions of the M1928A1 Thompson that were streamlined and simplified for manufacture during World War 2, first adopted in April 1942. Features include a non-finned barrel, simple fixed “L” style sights, no Cutts Compensators, and elimination of the knurling on actuators and fire select levers in later models. The bolt and receiver were simplified to eliminate the Blish lock “H” piece. The bolt actuator was also moved from the top to the right side of the receiver. Also there are examples that were parkerized rather than blued that ended up in the hands of U.S. Soldiers.