[The following excerpt is published courtesy of DLRC Press and its author, John Feigenbaum. This information was originally published in 1994 in The Complete Guide to Washington Quarters.]
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101 1970. Doubled die reverse.
102 1970-D. Doubled die obverse. Breen reports that many DDO varieties exist for this date.
103 1970-D. Doubled die reverse. (FS#25¢-027.3) (photo)
Type C reverse was used on all Philadelphia business strikes and primarily on the Denver issues. Breen reports that fewer than 0.1% of the Denver issues were produced with the type B reverse which makes them very scarce. As with the 1969 proofs, the type B reverse was used exclusively on the S-mint coinage.