S was for "speaker". the idea was to use it for guitar or bass, whichever you preferred. In general, cabinets with 12" or smaller drivers were used for guitar, 15" and larger for bass, but there were exceptions. For instance, John Entwistle used Coliseum heads with both 118 and 412 cabinets. Probably he opted for better drivers than the basic Sunn transducers, though. 118M or 118V cabinets, along with 412M cabinets, but I'm just guessing.