Albums of material scooped from cutting room floors usually confirm a band’s good taste in leaving it there in the first place. That’s the case with this after-the-fact LP of previously unreleased tracks by the mightiest of the heavy metallurgists. (The Zep disbanded after the 1980 death of drummer John Bonham.) Still, it must be said that Zeppelin even on a bad day provided more power and invention than most rock groups. The album is of greatest interest in tracing the band’s evolution, beginning with its London blues-rock roots, illustrated here by the 1969 song We’re Gonna Groove and a blistering live 1970 version of Willie Dixon’s I Can’t Quit You Baby. Poor Tom features the acoustic-guitar brilliance of Jimmy Page that made virtually all of 1970’s Led Zeppelin III so delicately stunning. Walter’s Walk, from 1972, prefigures the spare, garage-band energy Zep generated on transitional mid-’70s albums such as Presence and Physical Graffiti. There’s also a de rigueur Bonham solo from 1976, Bonzo’s Montreux, with Page’s bizarre electronic distortions. Three tracks were recorded at the same Stockholm studio where the band’s final LP, In Through the Out Door, was done in 1979; not surprisingly, on the track Ozone Baby, Page’s guitar sounds eerily like it does on that album’s All My Love. Except for Bonzo’s Montreux, there is no material here from 1973 to 1977, and that is unfortunate; it was a period in which Zeppelin flourished and moved toward its seminal fusion of heavy metal and New Wave.