From 1965 thru 1970, all Kennedy Half Dollars contained 40% silver. Also, in addition to the circulating, copper-nickel Eisenhower Dollars introduced in 1971, special collector edition Eisenhower Dollars were produced with a 40% silver content from 1971 to 1974. These 40% silver Eisenhower Dollars were all minted at the San Francisco mint, and carry the “S” mint mark below Eisenhower’s neck, but be aware that the San Francisco mint also produced copper-nickel Eisenhower Dollars with the “S” mint mark, which were sold in Proof Sets.
The 40% silver Eisenhower Dollars were sold in both an Uncirculated version and a Proof version; the Uncirculated coins came in a cellophane package inside a blue envelope, and are therefore commonly referred to as “Blue Ikes”. The Proof coins came in a hard plastic lens nestled inside a brown cardboard box, and are therefore commonly referred to as “Brown Ikes”.