When Eric Clapton created a void by leaving John Mayall’s Bluesbreakers, the leader rallied with a powerhouse core of guitarist Peter Green, drummer Mick Fleetwood, and bassist John McVie. The newcomers only played with Mayall for three months in 1967, and would go on to found Fleetwood Mac. But a bold Dutch 16-year-old, Tom Huissen, snuck a reel-to-reel recorder into five London clubs to capture these remarkable, previously unreleased performances. The sound quality on Mayall’s voice is a bit muddy, but the fidelity of the instruments is fine. Green, often overlooked in the era’s blues-guitar pantheon, steps up with a godlike display here. He blisters through three Freddie King tunes (notably an astonishing “San-Ho-Zay”), and adds an emotionally transporting tone on Otis Rush’s “Double Trouble” and aching bent notes on Mayall’s own “Brand New Start.” The rhythm section nails it all down with utmost precision. This is what some fans used to call “drinking music” back in the day; for some of us, it still is. (Out Tuesday)