Following up with some more date codes...

Cyclone MKII front derailleur is stamped XG, which is July 1981. The Pro-Am hubs that came with the bike are stamped “12 81” for Dec 1981.

My guess is that the only things the previous owner swapped out were the stem, bars, and brake levers, and the shop that did it probably kept the old ones, since the originals would not have been perceived as special at the time.