This site http://www.penceland.com/julcol6.html mentions this about 9/2/68:
"Santana was there, and the Grateful Dead showed up, but they weren't billed," Skok said. "By all accounts, the highlight of the festival was a monstrous jam with the Grateful Dead, Big Mama Thornton who wrote 'Hound Dog' for Elvis, James Cotton and Billy Roberts. "Billy Roberts' claim to fame was that he wrote the song 'Hey Joe.' " This isn't listed in deadlists nor is it on the source at archive.org. If the jam actually occurred it would be great to find it!
