Interesting someone at the studio at least thought the college and alternative crowd might "get" the movie. Not sure if "underground radio" is a polite term for pirate radio stations.
On the lobby card there appears to be an X rating. X was briefly used for mature content films like Midnight Cowboy in the U.S. (The only Oscar winner originally rated X, IIRC.)
But sometime after the ratings system was revamped slightly, (GP was dropped and R added) the X rating came to symbolize pornographic films alone. The MPAA ratings board abandoned the X and replaced it with NC-17, but the stigma never went away, and no major studio wants to release a film with such a rating. Many media outlets here refuse to carry advertising for NC-17 films.
That Lucas made a potentially X rated film is hilarious.