A quick look on Wikipedia (so may not be accurate
) gives the following:-
"The first aircraft to be certified to CAT III standards, on 28 December 1968,[1] was the Sud Aviation Caravelle, followed by the Hawker-Siddeley HS.121 Trident in May 1972 (CAT IIIA) and to CAT IIIB during 1975. The Trident had been certified to CAT II on 7 February 1968."
Therefore there could still be many diversions in the 70s if the vis was below the then current certified limits.