A highly talented trombonist, bluesy, fine soloist, Mr ''Knepper'' was a busy professional during his half-a-century career. A major asset of ''Mingus'' formations between 1957-1962, he's also the unfortunate part of a ''Mingus'' violent moment, which resulted for ''Mingus'' in a court assignment, and for ''Knepper'' in broken teeth and loss of an octave in his playing range. ''Knepper'' also recorded a number of sessions under his own name, and took, at the other end of his career, a strong part in ''Mingus Dinasty'' records.