Upload & Sell: On
Maxim's got it right. The Mamiya 645 50mm focal length lens will only give you the "31mm equivalent" wide angle view on a 5D if the 5D sensor is the same size as the M645 image, which it's unfortunately not, or as Hubsand says, if the entire M645 image is mapped to the 5D sensor, which would require optics in the adapter to compress the image. The existing adapters from Zork, Mirex and Hartblei don't contain optics (and IQ would suffer if they did). The difference between the relatively large image circle diameter from teh M645 lens and the relatively small 5D sensor size, means that there is a bunch of "spare" image circle around the 5D sensor. This allows you to shift the lens with respect to the body, and the 5D sensor is still well within the 645 lens image circle, and so no vignetting, until as Hubsand says, you hit the mirror box.
Here's two shots I just took with my 5D. The first is with a Mamiya M645 C 35mm f/4 N lens...