Harpo and Chico Marx and was broadcast on CBS's General Electric Theater on March 8, 1959.
At the very end, Groucho makes a surprise appearance -- uncredited and unpublicized, because he was still appearing on NBC in You Bet Your Life. Groucho delivers the only spoken line in the piece.
"The Incredible Jewel Robbery" turned out to be the final appearance of Groucho, Harpo, and Chico together. (Well -- as far as anyone knew at the time. Unreleased Marx Brothers footage from Philip Rapp's abortive sitcom pilot Deputy Seraph has since surfaced, and is widely available online. That was filmed in 1959, just a few months after "The Incredible Jewel Robbery," and two years before Chico's death. But we'll talk about Deputy Seraph another time if you'll just be patient.)