"KMET & Jim Pewter Present The First International Surfers' Stomp, Ten Years Later"
I have an original progarm from the event, which was held on August 26, 1973. The write-up on LMS mentions the recent vocal re-recordings with Bruce Johnston and Terry Melcher. It also refers to LMS as a further development of the band California.
This was before their official reunion, so they were not billed as Jan & Dean. Regardless of who may have been onstage with them that day, the band did not play live. Jan & Dean lip-synced to a recorded track.
Given Jan's condition, Dean tried to make a joke out of the performance. But the audience didn't understand the situation with Jan, seven years after the car accident. There was no explanation. They felt cheated by the lip-syncing and booed them off the stage.