The Penthouse letter author explained it; crotch(es) was/were/are grinding their clitori (clitorises sounds wrong) on his fingers resting upon the armrest.
This is indeed quite unusual. I have fond memories as a child feeling bosoms pillowed against my shoulder whist getting my hair cut, but not a single clitoris to be found.
Edit: I mean, I can find 'em now. I’m a veritable clit detective. I can find a clit in 2 seconds flat, boy howdy!