PART III. Die Installation Errors:
Mules:
Wrong hammer die
Definition: Mules are coins struck by mismatched dies. In this case, the wrong hammer die was installed in a coin press.
The 2005 Malaysian mule shown below was struck on a copper-nickel 10 sen planchet inside a reeded 10 sen collar and features a normal 10 sen reverse design. The obverse (struck by the hammer die) shows an unexpected and appreciably smaller 1 sen design. This resulted in a weakly struck periphery on the reverse face. A normal 1 sen coin is copper-plated steel.
This dual denomination mule was struck in 1995 on a dime planchet by a press that was striking dimes. The reverse die (anvil die) was appropriate while the obverse die (hammer die) was incorrect.
Images courtesy of Heritage Auctions.