The 1870-S Half Dime represents one of the most enigmatic rarities in American numismatics. According to official mint records no examples of the coin were ever struck, yet a single example was discovered in 1978. This represented a major numismatic event and added a unique date to the series of Liberty Seated Half Dimes.
The story of the discovery has been recounted in various references and although it seems fantastic, it has been confirmed multiple times by major numismatic scholars. The unique 1870-S Seated Liberty Half Dime was originally discovered in a dealer’s “junk box” containing common, low value coins. Shortly thereafter, it would be publicly exhibited at the 1978 American Numismatic Association convention held in Houston, Texas. The coin was examined by Kam Ahwash, notable expert on Seated Liberty coinage, and Walter Breen, major numismatic scholar, and declared a genuine.
The cornerstone of the second San Francisco Mint was laid in 1870 and it is believed that this event led to the creation of the 1870-S Half Dime, and the other unique 1870-S denomination, the 1870-S Three Dollar Gold piece. The information about these two unique pieces is based on a small slip of paper that accompanied the latter at its first public offering in 1911. The paper contained a note from J.B. Hampstead, Chief Coiner of the San Francisco Mint in 1870, indicating that two examples of the 1870-S Three Dollar Gold Piece had been struck, with one placed at the cornerstone of the new Mint and the second kept as a duplicate.
While unconfirmed, most numismatic researchers believe that the information provided in the note by Hampstead also applies to the 1870-S Seated Liberty Half Dime. If this is true, then examples of the two unique coins could still be retained within the cornerstone of the second San Francisco Mint, possibly along with other 1870-S dated coins of other denominations.