Not that there was a time when he wasn’t, but Charles Mingus, the extraordinarily gifted jazz bassist and composer, was in a particularly forward-thinking phase when he brought his latest ensemble to the Swiss Montreux Jazz Festival in 1975. The music is not experimental in the sense that it had gone so far out as to become inaccessible, but it was more that he was open to ideas that might have seemed of little interest to him five or 10 years earlier. That sense of expanded focus is demonstrated right at the outset of this two-disc, 90-minute set with “Devil Blues,” drawn—as are all three lengthy tracks on disc one—from Mingus’ Changes One and Changes Two albums of 1974–75. The track is a gritty blues, written and sung coarsely here by the band’s tenor saxophonist George Adams (words by blues great Gatemouth Brown), with pianist Don Pullen bringing to it a downhome vibe more fitting of a Texas roadhouse than a Northeast jazz festival. “Free Cell Block F, ‘Tis Nazi USA” swings like mad, giving trumpeter Jack Walrath and the rhythm section (Dannie Richmond plays drums) plenty of room to open up, while the half-hour-plus “Sue’s Changes” leaps giddily from steamy ensemble playing to the type of freeform expression guaranteed to drive non-jazz aficionados up a wall. For disc two, the band is joined by the great Gerry Mulligan on baritone sax and Benny Bailey on trumpet for a relatively sedate take on Mingus’ signature composition “Goodbye Pork Pie Hat” and the Billy Strayhorn standard “Take the ‘A’ Train.” For those who prefer their jazz more straight-ahead, these two pieces will likely be the highlights, but seasoned Mingus acolytes will relish (despite the dicey sound quality) the first part of the set.