A rather good recording of a lengthy rendition of Intersteller Overdrive. From the video description:

For five days in October 1969, , Belgium, played host to the Actuel Festival, an open-air music fest. Acts included Pink Floyd, Yes, Ten Years After, the Pretty Things, Keith Relf’s Renaissance, Captain Beefheart and a horde of bands you’ve probably never heard of.

Bringing a bonus layer of weird to the event was its master of ceremonies, the one and only Frank Zappa.

“That was after the Mothers [of Invention] had broken up, and y’know, I had time on my hands,” Zappa said several years later. “These people contacted me. They offered me $10,000 to be an emcee at a festival, all expenses paid, and go over there, and, y’know, whatever I wanted to do, and I said, ‘Fine.’ So, I get there, and they neglected to tell me that nobody spoke English.”

Zappa and Pink Floyd performed “Interstellar Overdrive” (an instrumental from Pink Floyd’s debut album The Piper at the Gates of Dawn, 1969 ) together during the Actuel Festival (1969) in Amougies, Belgium. However, when interviewed about the matter in They’re Doing the Interview of the Century, Part 2, Zappa denied this or simply couldn’t remember it. For decades fans weren’t sure whether Zappa’s jam with Pink Floyd was an urban legend or not? Photos and audio recordings eventually turned up, but it took until 2015 before video footage (and undeniable proof) of this legendary jam finally surfaced.

A bootleg recording of this concert exists: Pink Floyd Meets Frank Zappa.

“Frank Zappa is really one of those rare musicians that can play with us. The little he did in Amougies was terribly correct. But he’s the exception. Our music and the way we behave on stage, makes it very hard to improvise with us.” - Pink Floyd member Nick Mason, 1973.