@tubetime I can actually help with the 11695-502. It's a speed control chip, also used on the ST-225.

It looks like the Seagate custom motor driver the later-production ST-251 uses combines the Hitachi chip, the 11695-502, and the glue op amps from the early production board (as seen on that dubiously-correct M.I.T. schematic on BItsavers).