From Muppet Wiki

Burt Lancaster (1913-1994) was an Oscar-winning actor best known for his starring roles in From Here to Eternity and Birdman of Alcatraz.

He appeared in the first season of Sesame Street, saying the alphabet. The insert appeared in a number of episodes, beginning with Episode 0005.

Lancaster also appeared in a counting segment, reciting numbers while performing push-ups, which first appeared in episode 0010.