Plexes are first mentioned in 1960 https://dl.acm.org/doi/pdf/10.1145/366199.366256 https://dl.acm.org/doi/pdf/10.1145/366199.366256 and the paper even starts

Plexes are first mentioned in 1960 https://dl.acm.org/doi/pdf/10.1145/366199.366256 https://dl.acm.org/doi/pdf/10.1145/366199.366256 and the paper even starts with a critique of the efficiency of Lisp's approach for representing data with cons pairs (citing McCarthy's paper from the same year). You might also want to watch Casey's great talk on the history of OOP https://www.youtube.com/watch?v=wo84LFzx5nI https://www.youtube.com/watch?v=wo84LFzx5nI