The USA struck quarters, half dollars, and dollar coins in 1975, but all were dated 1776-1976 on the obverse and carried a Bicentennial design on the reverse. Obverse and reverse are numismatic jargon for heads and tails respectively. These 1776-1976 Bicentennial coins were also struck in 1976. The ordinary USA coinage resumed in 1977.

The Bicentennial coins were struck at three different mints: Philadelphia (no mint mark), Denver (D mint mark), and San Francisco (S mint mark). The Phladelphia and Denver coins were ordinary "business strikes" intended for circulation, and were struck on copper planchets (coin blanks) clad with a copper-nickel alloy. They contain no silver.

The San Francisco coins were intended for collectors and were sold at a premium. They exist as proof coins struck on copper-nickel clad copper planchets, as proof coins struck on silver-alloy clad copper planchets (40 percent net silver content), and as uncirculated coins struck on silver-alloy clad copper planchets. The proof coins are individually struck several times from highly polished dies on hand selected planchets. As a result they have a mirror-like surface that distinguishes them from ordinary uncirculated coins.

In addition to all this, there were two distinct reverse varieties used for the copper-nickel clad dollar coins. Variety 1 has low relief with bold lettering. Variety 2 has sharper relief with delicate lettering.

All together there are 5 collectible varieties of the Bicentennial quarter, 5 collectible varieties of the Bicentennial half dollar, and 8 collectible varieties of the Bicentennial dollar. For further information, get a copy of A Guide Book of United States Coins by R. S. Yeoman. It's the best general catalogue.