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.
It's a shame the adapters are out .
The M42 adapters I have all leave the lens aligned more or less where you would expect them to be .
Sounds like these adapters might be a bit of pot luck now .
It's simple to adjust the hood for adjustment , but AgX's suggestion might be worth a try if you don't intend adjusting it too often .
Put a mark on both the hood and body in two different colours so you can quickly adjust between the positions for both cameras without having to check each time .
Regarding aperture , IIRC I tended to use it at either f/8 or 11 for best image quality and didn't notice vignetting .