The Benglis reference refers to Benglis' infamous 1974 Artforum dildo ad, a double page ad of Benglis nude with a double dildo.

Ray Johnson (1927–1995) was the founder and foremost proponent of the New York Correspondence School, an essentially conceptual enterprise consisting of an underground network of collaborators who sent mail to each other.  Placing special emphasis on the structure of the postal system and the interconnections it enabled, Johnson is known for a strategy called “on-sending.” Like this Fredenthal correspondence, participants receive a letter or object in the post, add to or subtract from that item, and then mail it onward to another participant or return it to Johnson. 

Ruth Ann Fredenthal was an artist who also part of Ray Johnson's mail art network.