A possible reason for marking only the frame and muzzle plug/spring guide is that these are the only two components where perfect mating is important. When the muzzle cylinder plug and the cylinder had been threaded, the muzzle plug would have been screwed in fully and then the pivot hole drilled and barrel recess milled out to ensure perfect alignment. As it is very difficult to ensure exactly the start and end of the threading process it is most likely that if a different muzzle plug were to be used and screwed in fully, the pivot hole and barrel recess would not line up exactly.