“That’s why we call it the Workshop,” Charles Mingus says of his octet in the midst of multiple false starts on “Once There Was a Holding Corporation Called Old America” in this historic, once ultra-rare concert At UCLA. He pushes half the group offstage to “get this thing together” and leads the remaining quartet through a scalding “Ode to Bird and Dizzy” in which saxophonist Charles McPherson emerges a hero. When the full complement reassembles for “Old America,” it ends up the program’s highlight, a howling, exultant Mingus spurring the band forward. As with all great Mingus, the level of sympathetic interaction is near miraculous; everyone earns (and then some) the victory lap of “Muskrat Ramble” they take immediately afterward. UCLA is the composer with history on his mind, still irked at his dismissive treatment by management at the 1965 Monterey Jazz Festival where these compositions were originally to have been recorded. The long view is reflected in the urge to get the music right, with Mingus alternating between bass and piano, and in titles such as “Old America” and “They Trespass the Land of the Sacred Sioux.” What emerges on these tapes is a treasure chest.