It's easy to get confused but the bare bones are they are all basically the same gun from start to finish, and as the years went by they made longer and shorter ones, fiddled with the sights and triggers, and made them gradually simpler and cheaper to make
The later guns had a seperate trigger and sear compared to the early guns where the trigger acted directly on the piston rod, which may be a selling point, but the early guns are nicer IMHO with hand cut chequering and nice little touches like keeper screws / metal buttplates.