Three coins are dated 1975 and three “Bicentennial Preview” coins 1776-1976.
It’s a real conversation piece: the only U.S. proof set that contains three different dates. In 1975, the U.S. Mint didn’t issue quarter, half dollar and dollar coins for circulation. Hence the San Francisco Mint didn’t strike proofs dated 1975 in these denominations, to go with the 1975 cent, nickel and dime. Instead, the 1975 set included “preview” proofs of the copper-nickel Bicentennial quarter, half dollar and dollar, all dated 1776-1976. Finest proof minting with deep-struck engravings and mirror-finish fields. Its 2.8 mintage was 32% lower than the 1976 proof set’s.