Those WERE in use a long time, Alex. I hate to admit it, but when I first came to Vegas, I wasn’t even 21 and I played BJ in every casino and CC was the only one that proofed me. Afterwards, Circus Circus Manor became my hotel of choice in Vegas because those rooms came with a refrigerator (unusual at the time) and you could park your car right outside the window of your room. The hotstamps were there seemingly forever mid seventies well into the eighties, before the Clown series began.
The James Bond movie, ‘Diamonds are Forever’ (1971) has a scene there where Jill St. John plays 21. Perhaps that scene will yield some clues. First time I was there was 1975, and I never saw the Ringmaster series.