This song is about a Chicago groupie known as Cynthia Plaster Caster. When they were assigned an art project to plaster cast "something solid that could retain it's shape," Cynthia and her friends Dianne and Marilyn came up with a clever way to get backstage and have a unique interaction with band members (or band's members): they would made molds of their penises in plaster of paris.
Carrying around an official looking "kit," their plan worked and they succeeded in making casts for many musicians and their associates, most notably Jimi Hendrix, who had an exceptionally large cast. The Plaster Casters are discussed in detail in a chapter on groupies in Jerry Hopkins' book The Rock Story
, and some excellent reading is available at Cynthia's Website
. (thanks, Ekristheh - Halath)