Marilyn Chambers had just previewed part of her new stage show (left) at a New Jersey theater, and visions of topflight bookings were dancing in the head of her manager, Chuck Traynor. "She's only twenty-one," he told us in breathless non sequitur, "and she really explodes onstage." They were due back in Hollywood a few days later to see about some proposed movie roles. Which was the kind of action Miss Chambers had in mind when she left her staid New England home town and gravitated to New York. But while she modeled, studied acting and even played bit parts in a film or two, her career was going nowhere. Then she moved to San Francisco, where she answered a newspaper ad placed by the Mitchell brothers, stalwarts of the porno-movie industry. Marilyn wanted a nonballing role but settled for a nonspeaking one; the movie, Behind the Green Door, cast her as a beautiful abductec, submitting--and responding--to all kinds of bizarre sexual stimuli. But while it made the rounds, her past came back to haunt her: It was revealed that Marilyn Chambers, porno star, was the sweet young mother on the Ivory Snow, box in your supermarket. Procter & Gamble, which makes Ivory Snow, was mad; but in the long run, the incident sold a lot of soap--and gave Marilyn a boost, too. She followed Green Door with another porno hit, The Resurrection of Eve. and then Traynor--a sort of Svengali to female porno stars--entered her life. Now it looks like Marilyn, thanks to her sex-movie detour, may actually become the aboveground star she always wanted to be. Some may well regret that; but you can bet she isn't looking back.