If it was one single direct flight it can only have been BA or Qantas. Otherwise you must have changed somewhere, or conceivably Singapore Airlines with the same aircraft and a change of flight number in SIN. Quite a few European airlines flew to Australia in those days (but definitely not Sabena) but few if any of them served Perth. If anyone did, Lufthansa would be the most likely.
Dunno about everything being on the web, my office has a cellar-ful of old ABC schedule guides, we should have one for Winter 73-4, I'll see if I can dig it out.