This paragraph from Ralph J. Gleason's column in the August 25, 1969 San Francisco Chronicle says
Tomorrow night at the Family Dog on The Great Highway there will be a lightshow spectacular--The Great SF Light Show Jam--with 13 different light shows and taped music from three years of unissued tapes from the Matrix including tapes of Big Brother, Steve Miller, the Grateful Dead, Jefferson Airplane and Quicksilver Messenger Service
I had seen the Great SF Light Show Jam listed on various obscure flyers and thought little about it, since Light Shows are inherently of the moment. The idea that the Light Shows were performing to years of unissued live shows recorded at the Matrix--well, that's something else entirely.
Now, various Matrix tapes have circulated over the years, and I wouldn't be surprised if the ones that were used for the Family Dog show were the ones that we have already heard. Still, its a really intriguing thought. And the tapes must have sounded awfully good, not degraded (since so little time had passed) and blasted over a real concert sound system.
Sic transit gloria psychedelia.
Update: The event seems to have been repeated a month later (on Thursday, September 25, 1969). Given the description, I don't think new tapes were added to the mix (since other, non-Matrix, tapes were used also), but its still interesting to think about (the clip is from Ralph Gleason's SF Chronicle column, September 24, 1969).