I have similar Shimano cartridge BB (square-taper spindle, from 1995/1995) and the right-hand cup was permanently affixed (the side with the flange that prevented the BB from being threaded too far into the BB shell.)
That BB looks like the exact opposite of what i imagined-- it appears as if the adjustable cup had been glued/corroded in place and the fixed is clean as a whistle!
Weird.
How'd you remove it?