In 1960, not long after the release of his epochal Mingus Ah Um, bassist/composer Charles Mingus played one of Europe’s most venerable jazz festivals. The recording didn’t see release, however, until the mid-’70s, and what it documented was a band for the ages. The quartet played exploratory versions of Mingus evergreens, including the bluesy, politically charged “Prayer for Passive Resistance.” Innovative reedist Eric Dolphy subs for Booker Ervin on “What Love,” and Bud Powell, king of bebop pianists, joins for the standard “I’ll Remember April.”