most proofs(halves) you will find are not silver. It is very easy to tell, and when you get a silver one you will know immediately(if you edge check it). I have been doing a fair amount of volume for 9 months or so, and I have found 3 silver proofs, and probably close to 100 clad proofs. They are pretty self explanatory with the pre 1970 halves. Then they quit doing silver proofs up until 1993 I believe(somewhere around there). Once they brought it back, the silver proofs are 90% silver, and have a S mint mark. Yes, they still do clad halves with the S mint mark in addition to the silver proofs.
Did you find one?