https://www.premierguitar.com/gear/the-surprisingly-long-history-of-the-guitar-pick
The above link goes through the history of the guitar pick . The first company d'Andrea in 1920s bought in the celluloid picks and the popular shapes and back then were usually thin, medium, heavy . I think Fender and Gibson picks were made by them too.
The next company was Herco who eventually introduced the nylon picks in 60s. Many leading names used these notable the Flex 75 and flex 50 gold. Does not say when they were introduced, but maybe early 60s .
THIN picks were certainly around , possibly in celluloid and later nylon.
Bruce Welsh is the one to answer this I think, but I think he instinctively knew what worked best for his rhythm work.
Dunlop picks took over the market by 1980 with new materials. The article is worth a read. I always thought tortoise shell was the choice, but clearly everyone had their preferred choice, notable studio musicians had a trickbag of picks for different sounds.
I personally go for the Herco Flex 75 general picking also Flex 50 for rhythm.