I'm not sure Peavey made "S" Foundations in '85. They did make them in '86, though, so the seller could be a bit off. As far as I know, the "S" in the Foundation S is there to set the model apart from the regular Foundation bass, which had 2 Super Ferrite pickups. I don't know if "S" is an abbreviation for a specific word.
The "S" model Foundations are very good quality basses. If the price is right, it would be a nice buy.