Hiya........the three piece jobbies top left in the first piccy look like late type (1920's/30's) BSA standard ones.....if so, should be 28.4mm ish in diameter......I am only guessing this on having a couple of these guns with very similar units.......I also have one where there are two shallow holes drilled instead of a slot in the metal part.......Maybe way off the mark, but without knowing the size and threadform It's my best guess!