Here's a response I found by searching the archives at bikelist.org:
88 first year for RB1. Ishiwata CrMo tri-butted. Horizontal d/o. 73.5
st, 73.5 ht, 41.5 cs, 1005 wb
six sizes 48-51-53-56-59-63. Black/classic white and scarlet.
89 no name given CrMo tributted, same specs and sizes Scarlet/classic
white and Cobalt blue.
90 Ishiwata 022E quad-butted. vert d/o, 74 ht, 72.5 st, 41 cs, 995wb,six
sizes 47-50-53-56-59-62
black/white and Prism blue/Lilac
91 same specs, five sizes 50-53-56-59-62, Rich Red and Dark Green/Tusk.
92 Ishiwata 019E, 022E, 024E mixed for different sizes. same specs seven
sizes 50-53-54.5-56-57.5-59-62
Red and Yellow/white
93 same tubing, specs and sizes. Red 7-spd bar-cons, Yellow 8-spd STI
94 Tange CrMo butted. same specs and sizes. Red
As stated, Ishiwata is very similar to 531 and Columbus SL/SP, though maybe a smidge heavier, and thus stronger (i.e. better for heavier riders/longevity). I believe 019 was equivalent to the best offered by Reynolds and Columbus. The stays are a mystery, however.
That's a very nice frame with a nice paint job, IMHO. Nearly anything Bridgestone from the Grant Peterson era is regarded as classic, besides, it's lugged steel!