This date originally appeared in Deadbase as a Grateful Dead date. According to Blair Jackson (in outtakes from his Garcia autobiography), it was in fact the first New Riders of The Purple Sage show, which was described as “shambolic.”
Of course, John Dawson, Jerry Garcia and David Nelson had played as a trio in Menlo Park in May, and Garcia had played pedal steel guitar with various semi-acoustic configurations (including Bob Weir, Peter Grant and possibly others) at Peninsula School in Menlo Park (early June) and California Hall (June 11), but the Longshoreman’s Hall seems to be the first formal show of the five-piece NRPS (Garcia, Dawson, Nelson, Lesh, Hart).
This was some kind of Hell’s Angels party, and there were probably other acts, but they are unknown to me. I do not know if the band was actually billed or introduced as The New Riders of The Purple Sage. An August 1, 1969 gig at Berkeley's Bear's Lair was advertised as "Marmaduke, Jerry Garcia and Friends", but later August gigs were under the New Riders name.