I have a Yokota tandem. I don't know much either but the decals say "Yokota Project USA. There is a sticker with a picture of a bear that says designed and built in California, USA. There is another bear sticker that says true temper tubing 4130 or something like that. It seems like a pretty nice frame. I believe it is a 1992 or so. I know it's not much help, but it is all I can offer.
I too would be very interested in more info. if it become available.