As you're in Japan, if Mamiya samples are going to be anywhere, they're probably going to turn up there

Most likely it's an early lens for an m645. As for the stiff/sticky focus - I had that on a Mamiya 80/2.8 (also for an m645). Rather than disassemble the whole lens (beyond my skill set), I put a couple of drops of gun oil (a very light oil) down the focusing ring, and kept working the focus. Eventually it freed up nicely (although if you put too much oil, it keeps working it's way out again - a minor problem, really).

If you haven't already, and you've got an m645, I think you should grab that lens - good price. The 45mm has a great reputation. There are a lot of pics taken with them posted on Flickr. Although barrel distortion is minimal, it's still a bit too evident for my tastes, but if one jumped out and bit me like that, I'd probably take it YMMV.