This is a really excellent condition Mamiya 645 55mm f2.8 S lens. It is a superb lens.
This lens has slightly stiff focus when first turned then everything is fine – very common on Mamiya 645 lenses. The iris is perfect, and there is just normal slight paint wear on the mount. Optically it is as good as you will get, just about perfect glass with no issues at all.
Now seriously hard to find; this is a superb lens and you will not easily find a better example. This is the very highly regarded S model – the later much improved N version optics in a metal body – perhaps the best combination you can get. I use one oif these myself.