You could buy a 'universal' cable that had a brake terminus on one end and a shift terminus on the other end. Handy to carry as a spare on a long tour.
Well that's interesting. I wasn't aware of that or if I was then old age and dementia have addled my mind. <grin>
Since the bike was just acquired by me in 2010 it's probably very likely the cables weren't original to the bike made in 1978. Perhaps this better explains why they seemed the same size.