From what I read many moons ago, the extra hole was for a removable winder, kinda like what's used on the 2000FCW and 503CW bodies. The winder was never made, though.

The transitional bodies, also called 500C-C/M usually have serial numbers starting with the code UV or UH and, IIRC, came out in 1971, right before the official launch of the C/M body. For all intensive purposes, the transitional bodies are C/M bodies with the wrong model plate on them, so they wind up going for much less than the bodies marked C/M. The rear baffle doors sound the same as the ones on the C/M when the body is fired and the body has the same tabs that hold the screen in.

With respect to jamming the camera, it's very rare. I think I've done it once when I was checking out a 150mm Sonnar on my first 500. Mainly just someone being an idjit and doing things wrong. I did once have to unjam my old 500EL to get the 80mm off it, since the camera was functionally dead.