Not listed in DeadBase XI. The tapes circulating with this date are Jerry Garcia with Harvey Mandel (guitarist who played with Charlie Musselwhite and in Canned Heat), Stephen Miller (keyboards player from Linn County, Harvey Mandel's Band, Elvin Bishop's band, and The Allman Brothers Band).
Some tapes list this as also with Elvin Bishop on guitar. Unknown drummer(s). Miller may be the person singing on Feel It (title song for Elvin Bishop's second album), Three O'Clock In The Morning, and Mojo Woker. The jam after Mojo Worker or one of the unknown songs may be "Wade In The Water". The newspaper ads mention Jack Casady (bass player in Jefferson Airplane and Hot Tuna) so perhaps he is the bass player, but some tapes list George Chambers (bass player for The Chamber Brothers).
This jam has previously circulated as "Set 3" of 12/16/68, but Bill Gadsden says that it's "with Harvey Mandel from 12/24/68". From the San Francisco Chronicle newspaper of Monday December 23, 1968 pg 35: "At the Matrix tonight and tomorrow night: Jerry Garcia, Jack Casady, open end jam, no door charge" and elsewhere on the same page under the heading "OPENING TODAY": "ROCK CLUB - Jam Session with Jerry Garcia, Jack Cassady and others, from 9:30 p.m., the Matrix, 3138 Fillmore. Free." On page 26 of issue #34 (Summer 1996) of Dupree's Diamond News, Dick Latvala stated in an interview "There are tapes of 12/16/68 marked Hartbeats at the Matrix, but I don't know if that's really accurate. You know [David] Crosby was playing with them quite a lot that month."
He makes no other mention of 12/68 Matrix tapes in that article, but he has stated on another occasion that the tapes in the vault of 12/24/68 are labeled as Jerry Garcia and the Harvey Mandel Band.