The 1855-S Proof Three Dollar Gold Piece is one of the few branch mint proof coins with an undisputed status as such. Although uncustomary, during its first two years of operations the San Francisco Mint struck a small number of specially minted proof coins across a range of denominations.
The existence of this issue first came to light at the 1984 ANA Convention, and the coin has changed hands multiple times since then, sometimes through private sales and other times at auction. The coin most recently appeared at auction in August 2011 certified by NGC as PF 64 Cameo with a CAC verification sticker and realized an astounding $1,322,500.
Although a second example of this rare branch mint proof has been rumored, only one coin is positively known to exist.