These have turned up a couple of times in the past, always in Britain. The previous samples were entry level models with hi-tensile steel frames made by Kuwahara. You definitely have a better one as indicated by the Ishiwata tubing. Fortunately, Kuwahara used an open format serial numbers. Typically, they are located at the bottom of the seat tube, format yy mm xxxxx, where yy = year, mm = month.