Originally Posted by
Iowegian
I'm guessing this was a stop-gap measure until the new forging dies for vertical drop-outs were ready and the assembly process was ready to use the new parts.
I doubt it. Vertical dropouts have been available for many years before the OP's bike was spec'ed. Here's some SunTour examples from the late 70s:
I suspect it was a marketing decision to go with the then-fashionable horizontal dropout look.