I'd bet a small sum that that Birmingham mark is bogus. Second, if Japanese, I'd bet a fair sum that it is a very well made gun. Third, it is going to have to stand on its own Original Quality grade, which looks to be good, as it isn't going to get any help from the Brand Value of a maker's name.

Suggestion - get it vetted, clean it up, and enjoy it for what it most likely is - a nice, well made gun. Be very mizerly about spending money on it if you are concerned about resale value.