Concerning 3Rensho, Have you talked to Andrew Muzi who probably imported more of these bikes than anyone (am@yellowjersey.org) ??
If you want a suggestion for "The Most Appropriate" saddle on a 1983 3Rensho, I'd suggest an Elina Pro (an almost-identical twin of a Cinelli Unicanitor) Elina was one of the top plastic saddle makers in Japan. The Elina Pro was sometimes produced with bike manufacturer branding, such as "SEKAI Deluxe". The nicest ones were suede-covered. Elina made different models for BMX bikes.
Fujita is also good. They made a "Professional" plastic suede saddle (often labeled Fujita YFC) in cool colors, such as burgundy on the top-2 high-end 1982 Fuji models, and black slick vinyl on the Finest model.
http://classicfuji.com/
Another choice is Kashimax, but I don't know the model names in that era.
- Don Gillies
San Diego, CA, USA