Only one person in the world can have a complete set of United States coins. And the key coin of all is the 1873 CC "no arrows" dime -- only one of which is known. In 1873, the Carson City mint struck 12,400 such coins. But soon after striking, the weight of the dime was altered. The coins were to be melted and replaced by a slightly different version of the 1873 dime which contained arrows around the date, indicating the change in weight. Only one specimen seems to have escaped the melting pot. The unique 1873 CC "No arrows" dime has just been auctioned off in Philadelphia for 1.84 million dollars.
As for the seated liberty series itself, I, along with most collectors, find the series somewhat dull. I have always admired those collecting the series by date and mint. And for all of the collectors except the one who purchased this dime, there will forever be an empty space in the album.