Here is all kinds of color, from one John L. Wasserman column in the San Francisco Sunday Examiner and Chronicle Datebook from August 13, 1972:
Mike Bloomfield, Elvin Bishop, and Johnny Winter jammed together a few days ago at Keystone Korner. This interests me because I am trying to piece Summer 1972 back together (see Corry), and Bloomfield's movements are probably just as relevant (and more helpful, since he's more in the press) as the Merl and/or John Kahn movements. Of course, I am sure the music was good, though I actively don't love the first two, and have barely listened to the latter.
Pierce Street Annex has moved around the corner and now occupies the old Matrix club at 3138 Fillmore. This puts some time parameter around the Garcia-Guaraldi crossings at the Pierce Street Annex in the summer of '72 - presumably couldn't have been very much before this 8/13/72 publication (where "very much" might mean two weeks?).
There are also several Country Joe items in here, Yellow Shark take note.