First off, congrats on the silver bi. proof! Not the easiest of finds!
All Proofs come with a mint mark of "S", which you seem to already know. In 1976, there was a Clad S proof and a Silver S proof. 7 million clad were made and only 4 million silver ones. You have one of the silver ones.
There have been proofs marked with an S for kennedy halves all the way back to 1968, when the halves themselves were still 40% silver. From 68-70 is naturally silver, since the non proof version was 40% silver itself. AFTER 1970 though, the next silver half was the one you found... the 1976, and that was only a special edition one. The next silver S proof after the bi. that YOU found is the 1992 S. In 1992, the mint starting making a Clad S proof AND a Silver S proof. Much like they did in 1976 with the bi.
So, if you find an S proof from 68-70, 76-bi., or 92 onward through 08, you have a shot at having silver in the coin.
Hope that helps.
~Dave