At the Matrix on Thursday, September 4, 1969, Mendelbaum played with Womb.

By 1969 the club had become something different from the room that opened in 1965. Marty Balin had built it at 3138 Fillmore Street as a stage for the band he was assembling, and Jefferson Airplane settled on their name there on the first night; four years later Balin had long since left for the Airplane, and the Matrix was a proving ground for Bay Area groups who would not get a ballroom headline. Short runs, rotating support, nothing printed.

Mendelbaum were one of those: a local band whose album went unreleased for decades. Womb, sharing this night, were on a Fillmore West bill two miles away the same month, third under Spirit on David Singer's Michelangelo-on-the-moon sheet. The same second-tier bands circulated between the big hall and the small club within a single week, most of them a year or two from dissolving without a record anyone heard.

The Matrix outlasted both ballrooms — the Avalon closed in 1968, the Fillmore West in 1971, and the Matrix ran until 1972 — and is documented at a fraction of their resolution, because it never printed a poster.