I've got a M645J. To make this easy, if it is a Mamiya SEKOR-C lens mount, it will fit.
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Originally Posted by MattKing
Your perception may also be affected by the fact that 35mm film (and viewfinders) have a much more rectangular format than the 6x4.5. Wide vistas appear even wider in that long, narrow 3:2 format.
I believe you are correct about this. This is probably why it is more noticeable with 35mm cameras than with 645.