Ron Guth:
The 1851-O Quarter is a somewhat scarce issue, with a mintage of only 88,000 coins. In 1991, Larry Briggs identified two different die varieties that comprise the entire issue for this year; the two varieties share the same obverse. The reverses differ primarily by the position of the mintmark.
In Mint State, the 1851-O is a true condition-rarity with only a few Uncirculated examples available to collectors. The finest example is the PCGS MS64 that was once part of the Eugene H. Gardner collection.