From United-Mutations.com, as told by Frank Zappa:

“The day before the show, a journalist in New York City woke me up — knocked on the door and is standing there with a tape recorder and goes: “Frank, I’d like to introduce you to John Lennon,” you know, waiting for me to gasp and fall on the floor and I said “Well, ok. Come on in.” And we sat around and talked, and I think the first thing he said to me was “You’re not as ugly as I thought you would be.” So anyway, I thought he had a pretty good sense of humor so I invited him to come down and jam with us at the Fillmore East. We had already booked in a recording truck because we were making the “Live at the Fillmore” album at the time. After they had sat in with us, an arrangement was made that we would both have access to the tapes. He wanted to release it with his mix and I had the right to release it with my mix — so that’s how that one section came about. The bad part is, there’s a song that I wrote called “King Kong,” which we played that night, and I don’t know whether it was Yoko’s idea or John’s idea but they changed the name of the song to “Jam Rag,” gave themselves writing and publishing credit on it, stuck it on an album and never paid me. It was obviously not a jam session song — it’s got a melody, it’s got a bass line, it’s obviously an organized song — little bit disappointing. I’ve never released my version of the mixes of that night.”

YouTube footage of them jamming on this night:

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