The coin takes its name from its second owner, Phineas Adams, and its most famous modern owner, Amon Carter Sr, the former publisher of the Fort Worth Star Telegram. The purchaser of the coin is Bedminster, N.J., coin dealer John Albanese, who is no stranger to famous and expensive coins: a few years ago he sold a 1913 nickel for more than $4 million.
Eight of the 1804 coins were ordered struck by the US Department of State in 1834 to be given to foreign heads of state, including the King of Siam and the Sultan of Muscat. One specimen was made in 1857, possibly illegally, and the other six, including the Adams-Carter dollar, were made sometime after that, also possibly illegally. It is precisely this mystery and intrigue, not to mention the numerous famous owners and stories surrounding the coins, that make it collectively the single most famous coin in the world.
The Adams-Carter is the best 1804 coin outside the specimen currently held by the Smithsonian.
Another issue, the 1933 double eagle $20 gold coin, with an auction record in excess of $7 million, holds the title of most expensive coin, at least based on past sales. However, no other US coin is popularly accepted as the "King of Coins." The 1804 silver dollar is clearly the most famous coin ever struck at a US mint. Its rarity has been documented for more than 150 years.
Source: antiquesandthearts.com