In any other genre, the idea of an “unreleased classic” might be an oxymoron, but not in dubstep. In the 2000s, DJs routinely guarded their best tunes on individually pressed dubplates, and fan favorites might not see an official release until after months of club play. In the case of these heavy-hitting sides from Skream, aka South London’s Oliver Jones, nearly two decades elapsed before they finally saw the light of day. All eight cuts draw from the cauldron of dubstep’s early history; Skream wouldn’t release his debut single (“The Judgement,” with Benga) until mid-2003, and “Midnight Request Line,” his breakthrough, didn’t come until 2005. But his signature is all over these tunes, audible in darkly unfurling bass tones, squelching filters, and the faintest hint of dub echo. Dubstep had yet to settle into its standard half-time cadence, so the rhythms here are refreshingly diverse: “Kontrast” rides coiled forward momentum; “Future’s Dark” rides atop skanking reggae chords; “Konga,” with its 4/4 beat and rippling hand drums, uncannily anticipates the sound of UK funky, still several years away. And while “Depth Charge” suggests the atmospheric new frontiers that dubstep would soon open up, the closing “Oh My Gosh” is the opposite. With its chilly square-wave bass and brittle drums, it’s practically a grime tune, which makes this more than just a collection of historical curios; it’s practically a Rosetta stone of UK bass music.