Hello Peter,
Looks like others have already jumped in and answered your question. I'm pretty sure it was the 503CW, as FlyingCamera said, that introduced the gliding mirror. I've always been very happy with my old 500C/M so, to be honest, I didn't really keep up with the 5xx series as it matured.
All that said, though, I would'nt worry too much about the vignetting with a longer lens (like the 250), if you decide on an older model. The 250CF is the longest Zeiss lens that I own and darkening of the corners (more upper than lower) is really not an issue. I certainly wouldn't make a buying decision based on vignetting (or not) alone. I'd think about how I'm going to use the camera...do I need a winder, for example. If so, you're gonna have to go with a later model.
Good luck with your decision. Whichever model you decide on I'd bet you won't be disappointed.