Think "flat fares" went out in the 1970s.
Before that, IATA regulated fares and these were the same on all its member (mostly government-owned) carriers. There were just two classes, with first class (F) fares being double those in economy (Y). The industry was heavily regulated, to an extent that is unbelievable today.
Only the rich, the elites and airline staff (on staff travel) could travel like this, everyone else was on charters out of LTN.