I don't know of any way to determine year of production on an SKB, and the Japanese don't include date codes among their proofmarks. I just checked an article I did on SKB doubles about 6 years ago. The end of the Ithaca/SKB partnership was announced Jan 77, and the Mitsui period did not last long. So my guess would be it's a 1977 or 78 gun.
I once had my hands on a Model 300 (also pretty rare) that was choked M/F. Can't say I ever handled a 400 so choked, but I would guess there are probably a few out there--but darned few of either the 300, 400, or 480 in any configuration.
