Flex began in 1983, well after the careers of the top bodybuilders of the 1960s. The only reason Arnold was ever on the cover was because he became a movie star.
Oliva also got banned by the IFBB when he entered WBBG contests beginning in 1972.
Muscle Builder was the Weider magazine throughout the 1960s and 1970s. If you look up a bodybuilder's name in
MuscleMemory and look at the bottom of the individual's page you'll see all the mags he's been on. Oliva has been on Muscle Builder four times.