When London released the initial batch of Phase 4 Stereo albums, they also released mono versions (through the first 12 titles at least) under the "Sound 4 Monophonic High Fidelity" label. They featured P 54xxx catalog prefixes (i.e., the Exotic Percussion example shown here is P 54004 in mono, SP 44004 in stereo). The vinyl was likewise pressed in England but sported a green-and-white rather than red-and-white label. The liner notes for the example shown here are more or less the same as on stereo release, but the graphics show the multi-track recording funneling down to one track rather than two. It is unlikely that there are unique releases on Sound 4 -- all of them should be available on Phase 4 Stereo. So, unless you have an odd preference for mono recordings over stereo, the Sound 4 releases are of little interest. After all, the adventurous use of stereo was what these recordings were all about.

Exotic Percussion - London Sound 4 Monophonic