There were a couple of cool GT hybrid/"crossover" models in the early 90s, maybe 1991 which had 700D wheels (583mm bsd) rather than 700c (622mm) wheels. Pretty much the same size as 650B/27.5" (584mm). They also had horizontal dropouts on the fork, so you could adjust the front wheel position. Double bosses at both dropouts, 3 water bottle bosses and mid blade bosses. GT Tachyon and Continuum, Tange Infinity tubing I think.
Edit. In this catalogue:
Maybe USA only.
Also came with the cool GT flip flop stem.