I've only seen a couple of posts over on Oz about these.
Originally these came with the Tange TX-Super forks. Could have been Tange Tube-kits made by another Jap manufacturer (National = Panasonic).
They have the fluted seat-tube, double gusset, soild mount brake bridge fitting (like the Rhino III) and Giant dropouts.
That frame on ebay seems to fit the bill. Quite a rare thing