Repunched Mint Mark (RPM)
1960-D Lincoln Memorial Repunched Mint Mark
Washington Silver Quarter Mint Mark Varieties Images and Values: 1950-D/S

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Mint marks used to be hand punched into the die after it arrived at the branch mint and often errors came from this process. A mint mark might be punched out of place, crooked or to weakly so the mint employee would attempt to repunch it correctly. This is how we get repunched mint marks called RPMs, but this is not the same as doubled mint marks; doubled implies a doubled die and that happens during a different process of the minting process.

Another problem that could occur is a different style (font) mint mark being struck over another or the hand punch could be broken and punch mint marks that look broke or odd shaped.