Something ELSE from Matsumoku...?

Here's one of the jewels of my japanese collection - a mint NOS 1985 Washburn B-50RS Rudy Sarzo Sig. "Slammer" bass...

What I was able to learn was that Matsumoku was making Washburn instruments up until mid-1985 when the Kasuga factory took over Washburn manufacturing duties...

So, my B-50RS has a serial number which indicates that it was built in 1985 at the Kasuga factory which means that it is not a "Matsumoku build"...? 

Does anyone here have more knowledge of the Washburn Matsumoku/Kasuga switchover, the B-50RS (single pup version) and/or the B-70RS (double pup version)...?