You need one of the RB backs - the RZ backs won't work.
They come in three versions - Pro, Pro-S, Pro-SD.
Is your RB a Pro, Pro-S or Pro-SD model? And did it include the rotating back adapter? You will need the rotating back adapter to use the standard backs.
The initial, Pro version does not include all the interlocks that later versions have. So it is possible to forget to take out the dark slide, or advance the film before taking your next shot.
The Pro-S and Pro-SD bodies add those safety interlocks, but the interlocks only work if you use a Pro-S or Pro-SD back.
The Pro and Pro-S backs depend on foam light seals. The Pro-SD backs don't require them.
All three backs can fit on and work with all three models - it is just the interlocks that matter (mostly) when comparing the different versions.
Technically the backs consist of two parts - the outer shell and the inner insert - but almost all the functionality is on the inserts. The only thing to remember, though is that you need to match the shells and inserts. You cannot use a Pro insert with a Pro-S shell, for instance. Each of the shells and inserts are clearly marked as to which version they are.
Thank you very much for this detailed post.
Mine is RB67 Pro-SD, and i look for 6x7 back not 6x4.5 one, i will make sure i get the correct back, i saw some on ebay and keh, but i can'r decide yet on which yet.