Originally Posted by
Lascauxcaveman
My '82 MIJ Bianchi Limited (Ishiwata 022 SuperSet) has much the same legwork and a serial number starting in BS45...
I'm guessing a
SunTour MounTech rear derailleur would have come originally on that bike to go with the FD, if you want to go original. (Although an aRX-GT would be at least period correct and maybe hold together better). Everything else looks correct, AFICT.
Your Limited is actually a 1984 model. Bianchi didn't introduce their Superest frame design until 1984 and the S/N indicates February 1984.
Yes, the OEM rear derailleur was a SunTour Mountech but they had their issues, so it's not surprising that it's been replaced.