What is the difference between M1928 Thompson and M1A1 Thompson?

Answer 1

The M1928/A1 has the cocking handle on top, a finned barrel with the Cutts compensator, the "Blish lock", a bolt with an actual firing pin, and machined selector knobs. It can take the drum magazines or the 20 or 30 round sticks.

The M1A1 was designed to be easier and cheaper to manufacture and has none of these. The cocking handle is on the right side, the barrel is not finned and lacks the compensator, and there is no firing pin, just a pin-shaped projection in the bolt face. The selector knobs are much simpler. It also lacks the Blish lock, which was actually unnecessary. It cannot fit the drum magazines, which were, in any case, unsuitable for military use, as they are heavy, unreliable, and rattly.

The M1928's also all came with the vertical pistol foregrip, while most of the M1928A1's and all M1/M1A1'a had the horizontal foregrip. The buttstock on the M1928/A1's slides on and off and is locked with a button. The M1/A1's were fixed with two screws.