"The Great Rock 'n' Roll Swindle," the controversial film starring the Sex Pistols, is scheduled to screen tonight as the American Cinematheque's Rock on Film festival heads into its final weekend, but it won't be the "last U.S. screening ever , " as festival organizers have claimed. "It's impossible to say absolutely, but as far as we know, this is the last official screening," said American Cinematheque Artistic Director Gary Essert. However, Don Getz, head of London-based Playpont Films, world sales agent for the film, said, "I don't know the basis for that kind of a statement. That sounds like real b.s. to get people to come in and see (the film)." Getz said that Playpont gave permission for the one-time screening. (The only previous U.S. showing was at the Los Angeles Film Exposition in 1980.) The film's director, Julien Temple, was equally surprised at the news. "It sounds like hype to me. What are they going to do with the film . . . burn it?"