The grey outline is the mask. When I slacken the one screw that's accessible, I can move the mask around a little. When I do that, its position changes in the finder. I'm not saying something isn't decentred in the finder's optical path, however. The issue with the mask is that the screw goes into a slot that allows it to move front to back (up/down in the finder), but there's very little scope to move it left/right - the side of the slot hits the screw. The attached shows it after some adjustment - moved down and left a little when viewed through the finder, enough to get the missing rounded corners into view.
I did take the plate off that covers the parallelogram prism and had a look. There were packers at the top and on the right-hand sloping face (viewed from the front), and it all felt pretty solid. I could have a closer look to see if it might have become decentred, although I don't think I want to take it out unless I have something suitable to use as replacement packers (I suspect the original ones will disintegrate).