How about the 1870 S half dime
(unique), the 1870 S three dollar gold (unique but there may be a second in the S mint cornerstone.) Does an 1870 S quarter exist? they put a full set of coins in the cornerstone in 1870 and the quarter is the only one now that doesn't have a known example outside the stone. If they made duplicates of the half dime
, Seated dollar
and three dollar gold, they almost certainly did so with the quarter as well.
That is fascinating, another quarter could be out there and unknown to us.