Ron Guth: The 1905-O Micro O Barber Quarter was created when a die sinker used a mintmark punch intended for a quarter dollar which, ironically, is smaller than that used on the dimes. This variety was discovered by Howard Newcomb circa 1912 (Breen, 325); yes, that's the same Newcomb who is famous for his reference work on die varieties of 1816-1857 large cents.
This is a relatively scarce variety made all the more popular because it is the only major variety of the series. As of May 2011, PCGS has certified 68 examples of this variety, including five Mint State examples. The finest of those is a single MS-66.
Sources and/or recommended reading:
Breen, Walter. Walter Breen's Complete Encyclopedia of U.S. and Colonial Coins. New York: F.C.I. Press, Inc., 1988.
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