From the Nashville Skyline recording sessions, February 17, 1969 in Nashville, Tennessee. The video is probably from either the documentary Johnny Cash: The Man And His Music, released in 1979 or The Other Side Of Nashville, released in July 1984.
The Cash/Dylan sessions over February 17-18 generated only one commercial release on Nashville Skyline, a duet of Dylan's Girl From The North Country, which had originally appeared on The Freewheelin' Bob Dylan, but most - if not all - the material from the sessions have appeared on various bootlegs over the years. This excerpt from a comment on the Between Thought and Expression blog details an interesting history of the origin of the bootleg material:
"... A little history about the source: These were bootlegged in the late 70s or early 80s in poor quality. In 1985 or 86, my friend, Chris D., worked in a video store in Nashville (I lived near there). I had helped him get some space shot videos from NBC, so to return the favor, he called one day and asked if I wanted a cassette of the Cash session. He got it from a guy who found it in his attic, in a box marked "BDJC". That guy's father worked for CBS Nashville in 1969 and made a 1 inch mono reel-to-reel copy of the original session tape. He made Chris a cassette from the 1 inch tape. Chris copied the cassette, and gave me the original. In the fall of 1986, I went to a Dylan collectors meet in Chicago, and we daisy-chained 15 cassette decks together. I was second in the chain, and everyone past me copied my tape. By then, I had added the second "One Too Many Mornings" and the three songs from a betamax tape of the JC TV show, so this CD is one generation down from my original cassette. The original bootleg CD came out about two months later. "Mornings" has a tone shift in the middle, from the video when they change from the studio footage to the control room footage..."