The U.S. Mint revived the large dollar coin in 1971 with the Eisenhower series. At that time American coinage was transitioning away from silver content and a only small portion of these coins were output with 40% silver. Combined, more than four million Ikes were produced in 1972 by the Philadelphia, Denver and San Francisco Mints. About 2% of the year's total mintage contained silver, and those issues were struck exclusively by San Francisco in both proof and BU quality for collectors. You'll get both 40% silver 1972 proof and BU specimens for one low price.