Reference the Muppet Movie (1979).

A conceit of the film is that it tells the story “more or less” about how the Muppets got to Hollywood. Consequently, it has a present bookend narrative in which the Muppets’ “The Muppets Movie” gets screened. And in fact, most of the The Muppets Movie (1979) then is the flashback adventure of the in-universe Muppets’ movie.

On their travel, Kermit & Fozzie encounter a hitch-hiking Big Bird who has not yet started his Sesame Street career.

What year was the Muppets’ “the Muppet Movie” set within the Muppets’ universe?