McLean Stevenson began in show business at age 31. During the 1960s, he worked in night clubs and comedy cabarets, did summer stock theater and some television before moving to Hollywood, where he worked as a comedy writer for Tom Smothers. He eventually began acting in sketches. Mr. Stevenson is best known as the fumbling commanding officer, Lt. Colonel Henry Blake, on the CBS television series, M*A*S*H (1972). Over a period of thirty years, he also appeared on a number of television series and was a regular guest star on Johnny Carson's The …