I have only the 65mm finder. It works well, as does all my other Mamiya gear. In fact, I have found it works fairly well at the 6x9 portion for my 50mm lens on 6x7 backs.
I can't imagine why a 75mm finder wouldn't work for a 75mm lens. However, since you seem to have a specially built camera, there may be some reason it wouldn't.
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