Nearly a half-hour of unseen 8mm footage of the Rolling Stones, the Flying Burrito Brothers, Crosby, Stills, Nash & Young and other artists performing at Altamont has been recovered by the Library of Congress and published on its website, thanks to an unlikely find among 200,000 reels of film that were acquired 20 years ago.

The footage is silent and on the crude side by contemporary standards, and does not appear to have been shot by the documentary crew shooting the footage that ended up in the film “Gimme Shelter.” The portions that include the Stones’ nighttime performance are nearly too murky to make out, and there’s nothing in it related to the stabbing death of an audience member during that set that made Altamont go down in history.

Nonetheless, the daytime parts of the “home movie” will be fascinating to aficionados of late ’60s rock, offering close-up shots of some of the legends of that era from a position on or close to the stage, including glimpses of many artists that were never seen in the “Gimme Shelter” movie, caught in performance or just casually hanging out behind the stage.

According to a blog post on the Library of Congress’ website, the two reels newly put up for public viewing are believed to be “an orphan work, in this case abandoned at Palmer Labs by whoever shot it. They just never picked it up.”

Their acquisition dates back to 1996, when archivist Rick Prelinger bought 200,000 reels from Palmer Labs, a San Francisco film processing company, as it was going out of business, with no other interest than eventually scouring the non-pro footage for their interest as ephemeral films. In 2002, the collection was bought by the Library of Congress, which announced at the time that it “would take several years… to provide access to these films,” though archivists now note that it’s been 19 years and they’re still being gone through.

Recently, according to the blog, a technician came across two reels of 8mm reversal positive footage with handwriting on the film leader that bore the words “Stones in the Park.” That title was marked down in the inventory, and Mike Mashon, head of the Moving Image Section, became interested when he saw it, imagining that it might be a home movie of the Stones’ Hyde Park concert in 1969. The 2K digitization process revealed it actually was from Altamont and contained shots of multiple acts on the bill, including Santana and the Jefferson Airplane as well as the aforementioned groups.

“Although the footage is silent,” Mashon writes,” we were all thrilled to see close-up footage of concert performers who were cut from the (Maysles brothers’) film, such as Carlos Santana and Crosby, Stills, Nash, and Young. (CNSY wasn’t pleased with their performance and refused to let the Maysles include them.) It was especially great to see Gram Parsons fronting the Flying Burrito Brothers, since you only see the back of his head in ‘Gimme Shelter.’ Even better, there are good shots of Mick Jagger and Keith Richards off-stage watching him perform!”

The footage also includes plenty of crowd shots, including scenes where the air appears to be filled with mysterious projectiles. A commenter on the blog writes an explanation: “At 10:22 of the video those flying objects were plastic coffee can lids. We hitchhiked there and in the guys’ van were bags full of lids. We were far up the hill and started flying the frisbees into the crowd.”

Mashon told the Washington Post they still hope to learn who shot the footage, and to solve the mystery of how he or she had such good stage access to capture the apparently amateur footage. “If an owner emerges, certainly we’d be interested in hearing that. Somebody with proof. But as far as we know this film was abandoned,” he said. “If we had been able to track down a name we would have pursued that. But there were no clues, and the fate of the person behind the camera that day is unknown.”

Watch the 25 minutes of footage here.