Mule Mint Errors: The Rarest of the Rare
A mule error’s die pair possibilities can include the obverse die, reverse die, or collar die. It can occur using dies from the same coin series, but which were not intended to be used together (such as a pairing of two Roosevelt Dime reverse dies), or it can be dies from different coin series entirely (such as the famous 50 State Quarter obverse die and a Sacagawea reverse die).
In determining the denomination of a mule error (since frequently they are made up of differing denominations of coins, such as $1 and 25-cent coins, etc.), the coin’s denomination is considered to be whichever denomination’s planchet is used in the mule. So, for example, in the case of a $1 Sacagawea/50 State Quarter mule, because the mule is struck on a Sacagawea $1 planchet, it is considered to be a “$1 coin” and not a “25-cent coin.” If it had been on a quarter planchet, it would be described as a “25-cent coin.”
Full article: https://www.pcgs.com/news/mule-mint-errors-the-rarest
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