I've done some serious digging and like you said the info is pretty sketchy at best but I've pieced together from many sources what makes the most sense chronologicaly. From what I've put together "Kei" trained at the Fujigen plant in the 70's and left to start his own brand of guitars "Headway" in the early 80's. After 3 years and some success he decided to expand to basses (Riverhead) and released the Jupiter, Unicorn and Phantom line of basses. To my knowledge I only know of 2 pro players that used them Dave Pegg of Jethro Tull (Unicorn) and I'm not sure of his name but the bass player from 80's Canadian band Glass Tiger (Jupiter) were I first fell in love with that bass. Upon the release apparently by all accounts is when the first plant fire happened and they lost all their stock. A new plant was built and they invested heavily in Steinberg copys and Spector copys to try and get the offshore Asian market of either of these. It's well documented that Kramer bought Spector and kyboshed any Asian builds by moving production to the Czech Republic. Headway/Riverhead didn't have the capital $ left to get through the ressetion in the 80's and had no choice but to close. Most if the Luthiers then went to Baucus Guitars and a second plant fire "happened" this one seems a little more sketchy weather it actually happened. Many years later "Kei" moved to the Philippines and resurrected the name brand but not at all the same products. .. the RSB900 and RMB that you've mentioned were actually both from the second generation/plant not the first gen. I've found some Riverhead catalogs online and a bit of adverts that clearly only show the Unicorn, Jupiter and Phantom no RSB 900 or RMB or Riverhead Spector witch leads me to believe that they were in fact second gens. Here is a advert with Dave pegg... only the Unicorn, Jupiter and Phantoms are there.