Well I just had a look in my 645, the original one, the light absorbing liner behind the mirror and in front of the shutter appears to be a flock covered plastic liner. It could come loose, I guess and buckle and bow out on the sides, in front of the frame rails. If you can lock your shutter open on a B or T setting it's easier to look at. You do have to push in on one of the pins on the grip side of the camera to fool it into working without a film insert.

Those "marks" are on the sides of the frame and the camera, not the top and bottom.