Ah OK, my bad; It's just that the 35mm bayonette looked so much bigger… I just realized that due to the size difference, the rear of the 35mm is closer to the camera, and hence looks bigger due to perspective.
Anyway, "N" should not have anything to do with it. All Mamiya 645 lenses share the same 63.3 mm bayonette mount.
If it won't fit (does it fit but not turn? Not turn fully? Won't even fit?), then perhaps there's a physical defect with the 80/1.9, such as a bent flange, protruding screw, etc. Try to identify what is causing the snag.