Dub-plate

Their dub-plate (unreleased tracks on private pressings) battles are usually limited to JA [Jamaica] and the UK, but they have begun to invade the States with this unique form of entertainment.

Source: wiktionary

[A]t Duke Reid's Treasure isle studio the engineer Byron Smith had cut the dub-plate with the vocals accidentally turned down. After rocking the crowd with the original vocal pressing of the song, [Ruddy] Redwood played them this new voiceless version. The crowd went crazy, singing along, and he played the song so many times that night that by morning the acetate was worn out.

Source: wiktionary