Results from test today. Better, but a tweak is needed.
Zenitar 16mm Fisheye with rear-mounted yellow filter on a Mamiya MSX 1000. No adapter needed.
To my eye, the aperture scale landed at dead-top-center once fully screwed in. However, there is still a very small vignette in one of the corners on all shots. I think taking Neil's suggestion and loosening the screws to slightly turn the lens hood is the answer here. At that point, I'd probably dedicate the Zenitar to the Mamiya and call it a day instead of re-turning the hood for use on different cameras. Good to know that it can be done though if needed.
It would still be nice to know why the Zenitar overshoots things on the Minolta SRT-201! If the threads are wrong, I'm probably out of luck as the Minolta adapter was an ebay buy.