I love to find advertising material and ephemera that reveals some of the underground marketing of the sleaze paperback era.
These ink store stamps from "Rudy's Adult Books, Redondo Beach, CA" advertised an adult book exchange service in which $1 would get a "book just like the one you have just finished" sent to you "in a plain wrapper."
These stamps were inside a circulating copy of Sex in Prison (which I have to imagine was a disappointing title, most of the recipients being already over familiar with the practice).
The given address: 1505-1511 1/2 Aviation Blvd, Redondo Beach, CA 90278 shows a block of plain wrapper store fronts out of which Rudy might still be operating.
Find this book and more in my Vintage Paperbacks: Lesbian / Lesbiana catalog which has received a lot of recent additions (with more to come).