SEYMOUR, INDIANA TUESDAY, APRIL 13, 1971522-4871—UPI RadiophotoVietnam. The base, which is held by South Vietnamese troops, has been under assault for nearly two weeks and some sources believe a major fight is comma in the area around the base.Base Under Attack — An American gunner, center, holds his ears to protect them from the roar of his 175mm gun as it is fired against Communist concentrations around Fire Base 6 near Tan Cane, South