Chris
Slightly rose-tinted memories for the late 70s

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Being my usual pedantic self, just looked at my spotting logs for 1975-79. From a sample of 438:
Airbus A300 were around then (Air France, Lufthansa, Olympic).
Boeing 727, Boeing 737, DC-9 were plentiful. Tristars and DC-10s to a lesser degree. But Boeing 747 numbers (60) matched the 707s and 727s.
Of course, these were only new sightings, so any VC-10s, Tridents, BAC111 don't get included
For 1970-1974 (from a sample of 619), Boeing 707s are the biggest number (128) with DC-8s, DC-9s, Boeing 727 and Boeing 747 about half that. A300s were just starting to appear, still some Caravelles.
I reckon the comparative quietness of the 747 must have meant that it was not so obvious as the previous generation jets.
Although having looked at the "late 60s" data, that could be the era you are remembering - no 747s then!