Proof coins, by definition, are special numismatic items. They are produced specifically for collectors, given to dignitaries, or struck to commemorate momentous events. While proof coins are special as a category, a certain type is even more unusual: branch mint proofs. Until the mid to late 20th century, all proof coins were made at the Philadelphia Mint except for a select few. Proofs made at other facilities, also known as "branch mint proofs," are among the rarest of all United States coins.
In addition to being extremely scarce, many branch mint proofs are also mysterious in origin. In most cases numismatists can only speculate as to why they exist, as few official records exist. The 1838-O proof half dollars and 1839-O proof dimes, struck in New Orleans, were probably made to celebrate the opening of the mint and its first issuance of silver coins. The 1844-O proof $5 and $10 gold pieces, by comparison, are difficult to explain.
Branch mint proofs have also been made to celebrate endings, not just beginnings. For example, a unique proof 1907-D $20 was likely struck to mark the end of Liberty double eagles being struck in Denver. Similarly, this is probably why a tiny quantity of 1891-O proof dimes were made in New Orleans; this was the very last year of the Liberty dime before the Barber version was introduced.
Irrespective of the reason why they were struck, branch mint proofs are prohibitively rare and wildly valuable. In most instances just one or two pieces are known and, depending on condition, can be worth in the tens if not hundreds of thousands. A few particularly spectacular branch mint proofs, in fact, are worth in excess of $1 million.
Two of the most valuable branch mint proofs are unique double eagles. An amazing 1854-S $20, likely struck to celebrate the beginning of Liberty double eagle production In San Francisco, resides in the Smithsonian. The coin will almost certainly never be offered for sale, but if it were auctioned, would likely sell for multiple millions. Similarly, a unique 1856-O proof double eagle also exists, but its “raison d’etre” is unclear. Some speculate it was the first double eagle made in New Orleans that year. Regardless of why the coin was made, collectors consider it a highly desirable item. The piece traded hands for nearly $1.5 million in 2009, making it one of the most valuable Liberty double eagles in existence.