Riverhead basses were made by a company called Headway in the town of Matsumoto, so perhaps there is a connection. Certainly the quality of the Riverhead basses that I have played is at least as good as the Matsumoku guitars of that era. They're still around, still high quality, and still in Matsumoto, although the Riverhead brand is no longer the figurehead -- the company's now called Deviser, and their top-of-the-line instruments are released under the Bacchus brand. See
http://www.deviser.co.jp