The "silver series" of Washington quarters spans from 1932 to 1964; during many years in the series it will appear that certain mints did not mint Washington quarters for that year. No known examples of quarters were made in 1933, San Francisco abstained in 1934 and 1949, and stopped after 1955, until it resumed in 1968 by way of making proofs. Denver did not make quarters in 1938. Proof examples from 1936 to 1942 and 1950 to 1967 were struck at the Philadelphia Mint; in 1968 proof production was shifted to the San Francisco Mint.