Classes and "virtual" procedures were indeed already part of the original "Simula 67 Common Base Definition" in June 1967, but "protected" and "hidden" were add

Classes and "virtual" procedures were indeed already part of the original "Simula 67 Common Base Definition" in June 1967, but "protected" and "hidden" were added later (proposed here https://dl.acm.org/doi/abs/10.1145/956003.956009 https://dl.acm.org/doi/abs/10.1145/956003.956009, and probably added to the Common Base Language in 1982).