That is good question there, Lee. I have wondered the same things myself. To add to the mysteries, why did the mint wait until 1958 to use the proof reverse of the half on circulation strikes? And why did they stop after 1959? What were they thinking when they found a class III doubled die and buffed it to remove some signs of doubling before first use? The use of that doubled die, then, was deliberate and not an accident.
For 1971, 1972 look for the S mintmark. For 1973, 1974, 1976 look for the S mintmark then check for silver reeding. Cn clad proofs have a "bright" and clearly distinguishable copper edge.
hthttps://picasaweb.google.com/Ripley44mag/AmericanCoins?locked=true#5580024897223663218tp:// 1972 Silver Ike with S
oh. I was addressing post 28. hope that's OK. 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. Composition The Eisenhower Dollar was struck with a copper-nickel composition for circulation and was the first United States dollar coin to not be struck in a precious metal, although special collectors' issues were struck at the San Francisco Mint in a silver-copper composition. Copper-nickel issues: Weight: 22.68 grams Composition: Outer Layers of 75% copper, 25% nickel with a center layer of 100% copper Silver-copper issues (silver clad): Weight: 24.59 grams Composition: Outer layers of 80% silver with a center of 20.9% silver. Aggregate 60% copper, 40% silver Silver content: 0.3164 troy ounce (9.841 grams)