RWC is probably "rapid winding crank" as distinct from a winding knob. Re: the 500C versus C/M matter, the first C/M specification bodies were actually badged "500C". I can't recall if they are defined as 500C models updated to C/M spec; or rather, as C/Ms badged "500C"; I suspect the latter. The bottom line is they are the earliest version of the C/M with user changeable focus screen that was the major change from the previous 500C.
Being the earliest example of the ubiquitous C/M, and only made for a short time before the 500C/M bodies were also badged as "500C/M", they are, if not rare, then, a little less common than your typical body labelled "500C/M". Which makes them, if not collectible per se, then, at least, a little more interesting than later examples. I suspect that given a few more years they may be collectible to a degree; the dedicated Hasselblad collector would probably already have one on their list. If you just want a sound C/M to use there is no reason to buy one in preference to one that is badged "500C/M" but, equally, there is no reason not to buy one, assuming, of course, that it is in good condition. If I was seeking another 500C/M myself and an early 500C badged example in good repair presented itself, I would not hesitate to buy it.
Cheers,
Brett