Guild paid a lot of attention to its solid-body range in the early seventies, with several new models, like the S-90. Older models, like the S-50 were revamped - using the body style of the S-100.
Initially components were quite different - the guitars shown here have the sixties style Guild combination bridge, and although the S-90 has Guild humbuckers, like the S-100, the S-50 has Hagstrom single coils. By the mid seventies these differences were gone, and all three models had the same S-100 humbuckers, bridge and tailpiece. See the S-50 and S-90 in the 1975 Guild catalog.