Washington artist William A. Christenberry, 46, hadn't heard of the Lyndhurst Foundation. So he was surprised when, out of the blue, a man called to say he had won a tax-free $75,000 foundation prize. "He said it was based on my work up to this point. There were no strings attached." Christenberry asked if he should write a letter or something, but was told it wasn't necessary. "All we want to do is reward you for your work," the man said. The Chattanooga foundation, begun in 1978 from the estate of bottling magnate Lupton, gives grants and prizes in health, education and the arts. The grants go to organizations in the rural South, for example one in Kentucky that trains persons in film and theater as an alternative to the coal mines. The $75,000 prizes, which may not be applied for, are for people who have done significant work, found their stride and need money. The 1982 recipients were Marie Cerillo, a former nun doing educational work in rural Tennessee; South Carolina writer and historian Ted Rosengarten; filmmaker Ross Spears; and author Eudora Welty. Christenberry, who has a show opening April 26 at the Corcoran, said the prize makes him think, "Maybe there is something good going on somewhere."