By the way, some of this information is on the inside of the back cover page (essentially page 61) of the Mamiya RB67 Pro S instruction manual under "M-lock system accessories (for Mamiya Press)". That title may be a little misleading but what they mean is Mamiya Press accessories that will fit onto an RB body.
You know the really nice thing about 6X7? It will crop to 6X6 really easy! A lot easier than a 6X4.5! I shoot both the RB-67 and C330 and like them both. If I have a scene I envision looking good square I just crop it when printing. Yea I know you lose a few shots per roll but when I shoot my 6X7 I only shoot 9 because 10 don't fit in my negative holders.