This is the only known proof U.S. coin that shows two mint marks from two different U.S. Mints. It is a double denomination Bicentennial Quarter struck over a struck Lincoln Cent. There is one other proof Bicentennial Quarter double denomination known, which is struck over a 1969-S Lincoln Cent.

The 1976 Proof Bicentennial Quarters were struck at the San Francisco Mint. Somehow a Lincoln Cent that was struck in Denver was subsequently struck by proof Bicentennial Quarter dies at the San Francisco Mint.

1976 Bicentennial off-metal mint errors are very rare in every denomination struck for circulation. In proof, they are prohibitively rare with only 4 proof Bicentennial off-metal double denominations known on any denomination. There is a unique 1976-S Bicentennial proof quarter double struck on a struck dime, a unique 1976-S Bicentennial proof quarter struck over a struck 1967 dime, the 1976-S Bicentennial proof Quarter struck over a 1969-S Lincoln Cent mentioned above and this just discovered unique dual mint mark proof Bicentennial Quarter struck in San Francisco over a Denver minted Cent.

Proof coins are struck by technicians who hand feed the blanks into special presses. They are produced, examined and packaged using extreme quality control. It is very unusual to find major proof errors. A few broadstrikes, off-centers, double strikes (in collar) and off-metals have been known to be found in sealed proof sets. Proof errors are aggressively sought after by many error collectors.

This unique mint error offered here was just discovered and was authenticated and certified Proof 67 by NGC. It is unique and is a significant discovery for several important reasons. It is the only known proof Bicentennial Quarter double denomination with two different mint marks. It was preserved in amazing condition with considerable detail remaining on both the obverse and reverse. The reverse design of the Cent is mostly visible underneath the obverse design of the Quarter. The obverse design of the Cent is mostly visible underneath the reverse design of the Quarter, with the "D" mint mark visible between the M and E of AMERICA.

This discovery mint error is fascinating since it combines rarity (unique), quality (Proof 67), history (Bicentennial one year only design) and intrigue (the only known proof U.S. coin that shows mint marks from two different U.S. Mints). This enigmatic and unique U.S. proof mint error belongs in a major collection of the finest error coins or in a collection of Bicentennial coins.

Also just discovered is the only known proof Bicentennial Quarter struck over a Roosevelt Dime. In addition, it is a "dual date" since the Dime is dated 1967, an amazing 9 years apart! This unique double denomination was certified by NGC.

Exotic and Intentional U.S. Error Coins In The Marketplace One of the most controversial categories of U.S. coins are mint errors. Many dealers and collectors, as well as coin auction houses, buy, sell, trade and auction many rare, exotic and unique major mint errors. Obviously, some of these defy logic and were intentionally created and taken out of the Mint. In the early 2000's, a group of several hundred U.S. error coins were found in a safe-deposit box. Fred Weinberg purchased this group which included coins struck for proof sets and also coins struck for circulation. This group was auctioned by the California State Controller's Office of Unclaimed Property. The U.S. Secret Service inspected and released this collection to the State of California determining that it was legal to own. The State of California then auctioned the collection and the rest is history. Another example of U.S. error coins escaping the Mint occurred in the 1970's. A hoard of proof error coins were smuggled out of the San Francisco Mint inside the oil pans of forklifts that were being serviced outside of the Mint. This topic was discussed in the June 6, 2022 Issue of Coin World, which covered Fred Weinberg's account of this story. The Coin World Managing Editor concluded: Obviously, the marketplace accepts these coins, and some collectors are happy paying thousands of dollars for coins that show every indication of having been created through illegal means. In Episode 11 of the PCGS video series Slab Lab, Seth Chandler interviews Fred Weinberg. In part 2 of the interview, Fred explains in detail why mint errors that are decades old are not recovered by the U.S. Mint. Fred's recollection of conversations in his office with the Chief of the U.S. Mint Police are extremely insightful and explain why the Mint doesn't attempt to recover error coins from decades ago.

Proof Errors are featured in the NLG award
winning book, World's Greatest Mint Errors.