Some S mint marked Ikes are 40% silver. Here's an article from wikipedia.
Silver Issues
Some Eisenhower Dollars were minted in a 40% silver clad to be sold to collectors. All of these coins were minted at the San Francisco Mint, with dates 1971, 1972, 1973, 1974, and 1976. There are also approximately 15-20 silver clad coins in the 1977 variety. These coins were either uncirculated or proof. Uncirculated coins came in cellophane with a blue plastic token in a blue envelope. Proof issues came in a proof set-like plastic case, contained in a brown "wood grain finish" slipcase box with a gold seal on the back. The uncirculated coins are referred to as 'Blue Ikes' and the proofs as 'Brown Ikes'. Coins minted in 1975 and 1976 for the Bicentennial come with the quarter and the half dollar of that brief series. The uncirculated coins were sold by the Mint for $3; the proof versions for $10. Two varieties of the Bicentennial dollar were produced in 1975 and can be distinguished by the thickness of the lettering on the reverse. The Type I has thicker lettering, shown in the picture above, while the Type II has more delicate lettering. The Type II version is more common.
