Old Bushwhacker....
Thanks again, T-Mar.
Digging into this, I perhaps have found some more 'triangulation' info to add. The derailleurs, brakes, and levers are all branded Shimano Exage 'mountain LX'. From a quick Google, the 'mountain LX' line was only available starting in 1989 (and one site says only for that one year..?).
So maybe the frame was built and painted in '88, but the bike actually finished in 1989....?