Via POPMODERNE. According to the Muppet Wiki (and can I just take a moment to thank the Internet that such a thing as a Muppet Wiki exists): "Henson began the project in the spring of 1964 and continued to work on it for nearly a year, between commercial projects and various Muppet television appearances. The short film premiered in May 1965 at the Museum of Modern Art and was distributed through Pathe Contemporary films to arthouse theaters and the film festival circuit. It played in New York City along with the French feature A Man and a Woman."
Frank Oz and other Hensonites make appearances as well.