The transition from Ska to Rock Steady happened, inexplicably, in 1966. The standard explanation is that the weather was so hot nobody would dance to the upbeat Ska any more. Fair enough. But the lyrics got more political, and so the music itself became more sinister
The Melodians: Sweet Sensation. From the LP "The King Kong Compilation",
Island Records, 1981. Produced by Leslie Kong in the late 60's.