Some quick comments on that...
I also own a screwback pledge pin, I don't have a screwback service pin, so I would bid on that. For the age, $20 isn't unreasonable at all for a service or pledge pin. The screwback was used at least through 1952, since a brother who was a founder of the chapter at Catholic U. (Zeta Mu) in 1948 and he only remembers them looking like that.
Second. I know a number of fraternity/sorority pin collectors, some are even members of Fraternities or Sororities. I have no problem with one of them purchasing these pins, but still would prefer if a brother got them. Its not like a 20 year old is going to be walking around campus claiming to be a problem using this pin.
Not totally sure if it started out screwback or not, but given that it was a label pin from the beginning, that is reasonable.
Prior to 1939, I *think* the fraternity used as a pledge button, a shield divided into a blue left half and a gold right half. At least that's what is in the Baird's Manual of American College Fraternities in the 1930s. But I'm not sure how widespread that usage was.
Randy