I recently bought a used Mamiya 7 in immaculate condition except for the little key on the bottom that is used for closing the light curtain to permit changing lenses. It's flimsy plastic, and mine is broken. It's still usable, just, and I've no doubt I can get it replaced by a repairman.
I'm wondering though, if anyone knows a third-party supplier who makes those out of metal that would be more rugged than the one that comes with the camera. Surely I'm not the first person to have this problem.