Proof sets 1975: 2845450 sets minted by San Francisco mint. The quarter bore the 1776-1976 date
Proof sets 1976: 4149730 sets minted in San Francisco. The quarter bore the date 1776-1976
Silver-clad 1776-1976 quarters, San Francisco mint, abt 15000000. Nickel-clad ditto, Denver and Philadelphia mints, about 1.7 billion. Also about 7 million nickel-clad quarters from San Francisco.
The above from "A Guide Book of United States Coins."