hmmm...not too many Japanese frames with Campy DOs and forkends. The BB shell looks kind of middling and maybe has a welded seam, the fork crown looks a lot like a Tange crown(any stamps on the steerer?) the other details look pretty nice: lugwork and fillet brazing of the brake bridge, etc.: very tidy.
I couldn't rule out Japanese but the serial number sequence doesn't ring any bells
What size seatpost?