Can these be possibly correct?
6x7 is 56mmx69mm (Pentax standard and Mamiya is a hair larger).
So, talking only for the short sides, the lens factor or ratio is 56mm/36mm=1.56.
Your 50mm lens becomes a 78mm but with little bit longer (1:1.33 vs 1:1.25) view.
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Well, it doesn't really matter what the original format of the RZ is since the question was related to using a 36x48mm digital back. In that case the lenses behave just like on a 645 camera. Since the chip is smaller than 645 film ( multiplier 1.15 ) the lenses behave like on a 645 camera with that multiplier.
A 50mm lens is a 50mm lens on any camera ( 6x7, 645 or 35mm) It's the field of view that changes in function of the size of the capture medium. In case of 35mm format you could measure either the shorter or longer side to make a comparison, depending on what's is more important to you.
In portraiture a 4x5 vertical format makes visually more sense while in landscape and architecture photography a 3x2 horizontal format is visually more pleasing.
I.e. when I use my Canon 1Ds with a 80mm lens for a vertical portrait in the studio, I get the same framing as with a 110mm lens on my AFD 645 or Mamiya RZ with an Aptus Back. ( minus the top and bottom part of the 35mm frame).
If you would stick an old PhaseOne H10 back on a Mamiya RZ with a 50mm lens you would get the exact same field of view as with the 1Ds and a 50mm lens since that sensor is 24x36mm like a full frame 35mm.