When did Kermit become a frog?

Some sources, such as Jim Henson: The Works, mention that Kermit the Frog first became a frog in the 1971 special The Frog Prince. However, he was already a frog in the 1970 specials Hey, Cinderella!, The Muppets On Puppets, and the first season of Sesame Street which was filmed prior to The Frog Prince.

Kermit's first appearance as a frog was: ???