MMario appears to be correct (except, of course, that the date appeared as "1776-1976"! *G*).

One site about the Washington Quarter says, "There have only been two significant changes in the [Washigton Quarter] series since its inception in 1932. In 1965, the composition was changed from 90% silver to a clad or "sandwich metal" of 75% copper and 25% nickel, bonded to a pure copper core. The second modification came in 1975, when Jack Ahr's Bicentennial "Drummer Boy" design appeared on the reverse, with the dual-date 1776-1976 on the obverse. In 1977, the regular design returned." The entire article can be seen here: http://www.coinsite.com/CoinSite-PF/PParticles/25cwash.htm