I'm no expert but from my experience....
I'm currently running a box that is a replacement for the box that came with my ~'52/3.
I swapped the outer covers complete for some reason (maybe because I'd had it polished), it fitted straight on no problem, but wasn't until recently I realised the difference; the earlier outer cover (Rigid?/ early Plunger?) which accommodates the flat spring has a peg(roll-pin) that the ring on the end of the spring fits over, this peg doesn't go through the outer casing as does the later Plunger case which has a 1/4" Cheese-head (or whatever), and a nut on the outside.
As someone pointed out a while ago, the earlier main casing has a slightly different casting where the two bottom rear mounting bolts screw in, the late ones are a bit beefier but still fit ok, and I'm not aware of any other differences, except that swingarm cases are More or less completely different.
Hope that's helpful.
