According to the P&A's, the Corvair retainer went through several changes. It started out as 9775412 (1965 only), then went to 389809 (1965-1966), then 3897534 (1965-1968) and finally to 3934260 (1968-1969).
It's also known that the 3934260 retainer went though a design change from metal to plastic, but it's unknown exactly when. Early 70's Camaros have been observed with metal retainers while later Camaros have been observed with plastic. It also shows up in the later P&A's as "plastic".
However, for 67-69 Camaros, the round plastic retainer (3909199) is the one that's generally accepted as being "assembly line correct". But if you look in the 67 AIM (UPC 11-13, Sheet B4, Item 3, Note 4), it was changed from 3907534 (the 65-68 Corvair retainer) on 5-15-66. Was this a metal retainer or a plastic retainer? And did they ever make it into any 67 Camaro?
Again, retainers were supplied by multiple vendors, so after all these years, who knows!
Ed