I've heard the "US Import" argument lots of times, but I'm pretty sure it is incorrect.
The Americans got 17-digit VINs earlier than anyone else, the Japanese got them later that anyone else.
Through the late 80s and the 90s, a lot more "grey import" dirtbikes came from Japan than the USA.
The Americans refer to them as "Japanese numbers" (including the bloke who posted up the info on Thumper Talk) - it is certainly bloody difficult to find an English language frame number decoder for non-VIN Hondas made after the early 1980s.
As best as I can tell, Australian-delivered Hondas mostly had 17-digit VINs, but at least some (possibly later shipments) came with old-style frame numbers.