Currently the PCGS Pop report shows 8 certified 1851-O in MS66, however this is not truly correct as this example had been submitted 4 times in hope of reaching 67 status….by todays current standards the 67 designation would probably be attainable!
Ron Guth: The 1851-O is the only Three Cent Silver struck outside of the Philadelphia Mint. It is easily the scarcest of the Type One Three Cent Silvers, but it is more common than all of the Type 2 and Type 3 Three Cent Silvers except for the 1861 and 1862. However, demand for this date is high because of its mintmark. Mint State examples are fairly easy to locate, but most will be in the MS-63 and MS-64 range. Gems start to become scarce and none have been graded finer than MS-66 by PCGS (as of May 2011) except for the single MS-66+ in the unrivalled Simpson Collection. Most high-end examples of this date feature colorful toning and deep, frosty luster.
Jaime Hernandez: This is the only Three Cent coin struck by the New Orleans Mint. It is also the only coin that has a mint mark. The 1851-O is the toughest coin in the Type 1 series as it has the lowest mintage in the Type 1 series or for coins with the small star design. In uncirculated grades it is also the toughest coin as this is the only coin in the Type 1 series with no coins graded higher than Ms66 condition.