unworthy1, based on what I've read in some of Skip Magnuson's posts, this high 400,000 serial number should put it in '76 or '77, which would make sense for an early Japanese model.

The seatpost diameter is 26.8 mm, and the B/B shell width is 68 mm with standard ISO threading.

I should also mention that the rear driveside dropout had the derailleur hanger shaved off by a previous owner, in case anyone uses it's absence as a means of ID.