"But this impression might have been fostered by the peculiar and apparently permanennt twist of the arms of Sixties era screwballers Luis Arroyo, Bud Daley and the like. Their pitching arms when not in use did not hang straight down but were turned so that the palm of their hands faced to the rear."
My mid- and late- 1960's Dodgers had a great reliever named Jim Brewer who threw the screwball a lot. Vin Scully used to comment on the phenomenon you refer to, how this pitch had a permanent effect on how his arm "hung."
One of the amazing things about Fernando Valenzuela was the sheer number of screwballs he threw both in individual games and in his career. The guy would routinely throw more than 130 pitches in a start, and it wasn't uncommon for him to go over 150. IIRC he once threw over 170 pitches. A good percentage of his pitches were screwballs. And he threw this way for ten straight years AFTER having done the same in the Mexican League prior to MLB (how many years he did so, and the difference between his stated age and his actual age were the subject of intense rumor and speculation).