The population of certified George Washington State quarter dollar obverse/Sacagawea dollar reverse double-denomination mules has grown by one, to 15.

And if you've been following the story of these errors over the years, you won't be surprised to hear who owns it.

Professional Coin Grading Service has graded and encapsulated as Mint State 66 what is now the 15th known example of the (2000)-P error. It bears the obverse of a George Washington, State quarter dollar and reverse of a Sacagawea dollar.

This newly certified piece is identified as an example struck from Die Pair 1 from three known die pairs used to strike the errors.

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The newest piece reported has been acquired by New Mexico collector Tommy Bolack. With his latest purchase, Bolack now owns 11 of the 15 publicly identified examples of the mule.

The name of the seller, the price paid and details of where the error was found were not disclosed.

What is a mule?

A mule is a coin, medal or token struck from dies not normally intended to be paired together. It derives its name from the animal that is the offspring of a male donkey and a female horse.

The double-denomination mule error pairs the George Washington obverse from the State quarter dollar series with the Soaring Eagle reverse of the Sacagawea dollar. Each side bears a denomination and the coin is sometimes called a $1.25 piece.

The Washington obverse bears the P Mint mark but the 2000 date is missing; it would have been found on the correct mate for either of the mismatched pieces. On the State quarter dollar series, while the Mint mark is on the obverse, the date of issue appears on the reverse bearing each commemorative State quarter design. For the Sacagawea dollar, the date would appear on the obverse.

The mules were struck at the Philadelphia Mint on manganese-brass clad dollar planchets between dies fitted into a coinage press dedicated to Sacagawea dollar production.

The error type was first discovered in May 2000 in Mountain Home, Ark., by Frank Wallis, in an Uncirculated 25-coin roll of Sacagawea dollars from First National Bank & Trust. The Wallis coin was initially certified and encapsulated by Numismatic Guaranty Corp. as grading MS-66, but a month later, was crossed over into a Professional Coin Grading Service slab, also as grading MS-66. 

Three die pairs

Hobby experts by the fall of 2000 had determined that multiple die pairs existed for the coin. The presence of more than a single die pair would suggest that production of the errors was prolonged and the initial mintages sizeable.

Here’s how to distinguish the three die pairs:

??Die Pair 1: The reverse for Die Pair 1 exhibits a die crack in the F in OF in UNITED STATES OF AMERICA that is absent from the reverses from Die Pairs 2 and 3. The obverse exhibits numerous radial striations attributable to stresses involved during striking, resulting from the slight differences in size between the two dies. 

??Die Pair 2: Die Pair 2 exhibits a perfect obverse die, but a reverse that shows three noticeable die cracks: one each projecting from the rightmost points of the stars above the E of ONE and D of DOLLAR and a third, curved die crack running along the wing directly above these two letters. 

??Die Pair 3: For Die Pair 3, the obverse has been described as “fresh and frosty.” The obverse of the Die Pair 3 coins shows just a hint of the radial lines found on the discovery example. A small die gouge appears in front of Washington’s lips. The reverse appears perfect and exhibits no die cracks.

Cornering the market for the mules

Bolack has spent hundreds of thousands of dollars acquiring the error coins. The highest publicly known price for any of his mule errors is the $117,500 he paid to acquire a Numismatic Guaranty Corp. MS-67, Die Pair 1 example from Stack's Bowers Galleries' Aug. 6, 2014, auction.