The coin shortage of the early 1960s and a rising silver price ended the silver quarter. On June 3, 1965, President Lyndon Johnson proposed taking silver out of the dime and quarter, and the Coinage Act of 1965 followed in July. The new quarter had copper-nickel faces bonded to a core of pure copper. John Flanagan's design stayed, though Wikipedia notes that the Mint lowered its relief on both sides for the clad coinage. The silver quarters of 1932 to 1964 are covered on the Washington silver quarter page.
No quarter dated 1965, 1966 or 1967 has a mint mark. NGC says the marks were suspended to discourage speculators, who were blamed for the shortage. The 1965 date stayed on the dies well into 1966; Wikipedia reports that 1966-dated quarters were first struck on August 1 that year. Proof sets stopped as well. Collectors were offered Special Mint Sets instead, made by the San Francisco Assay Office, with a finish better than circulation strikes but short of a proof.
Mint marks returned in 1968, now on the obverse. Denver added a D, while San Francisco struck quarters only for collectors, almost all of them proofs with an S. Philadelphia's quarters stayed unmarked until it adopted the P in 1980. West Point struck some quarters from 1977 to 1979 without any mark of its own. Wikipedia also records a small change to the obverse in 1974, when some details were sharpened.
For 1975 and 1976 the quarter had a colonial drummer reverse and the dual date 1776-1976, so no quarter is dated 1975. Those coins, including the 40% silver collector issue, are covered in the 1976 Bicentennial coins guide. The eagle reverse returned in 1977, and the Mint engravers adjusted the design several times over the next two decades.
Silver came back for collectors in 1992. Public Law 101-585, signed November 15, 1990, authorized 90% silver proof sets, and Wikipedia says the first sets slipped to 1992 because silver blanks were hard to obtain. The 50 States Commemorative Coin Program Act, signed December 1, 1997, then replaced the eagle reverse. 1998 was the last year of this series, and the State Quarters began with Delaware in 1999.
