Notice that in that picture, there's a single sear notch on the bottom of the bolt. That was standard, but it meant that the safety could not be applied when the bolt was in the forward position. If the bolt was in the forward position (uncocked) and a loaded (stick) magazine was inserted, the gun could theoretically fire if dropped on the butt. The British noticed this and milled a second sear notch in some of their Lend-Lease guns so that the bolt could be locked in the forward position. I believe the M1 / M1A1 Thompsons came from the factory with two sear notches.