Jerry Garcia and Merl Saunders were advertised to play the Matrix at 3138 Fillmore on Wednesday, March 3, 1971. But as far as I know, GD played a benefit that night at the corner of Van Ness and Market.
Indeed, the same-day SFC listed both, which is a neat reminder that maybe whoever was taking the calls for the "Something Else" listings under Wasserman's byline might not have known that Garcia was in the Grateful Dead. Occam's Razor suggests just a basic mistake, but the former idea tickles me more.
I wonder if Merl played the Matrix anyway? He had played there a week and a half prior with Carlos Santana, but Carlos was already in Ghana for the big gig in Africa. I wouldn't have minded hearing what Saunders-Kahn-Vitt would do as an organ trio, had I been down in the Marina this particular midweek. But there are lots of other interesting possibilities, which I invite you to speculate about in comments.