A question: what do you want to use it for ? Architecture ?
Please keep in mind that a 75mm on a RB is a verry moderate wide-angle.
In architecture you will need something like a 50mm shift and Mamiya does not make such a lens.
If you want to make top-notch interior shots: look at a small TC with a roll-film back.
With that camera you have more lenses to choose from, all with a shift capability.