A re-punched mint mark, or RPM, is a coin that shows two or more mint marks on the same coin. This was caused by human error in the punching of the mint mark onto the coin die . Before 1990, a U.S. Mint engraver manually punched the mint mark into each individual working die.
Occasionally, due to human error, a die would get two or more punches of the same mint mark, sometimes in almost the same location, and sometimes at 90 or 180 degree rotations. Some times the Mint engraver would catch these defective working dies before any coins were produced from them.
A working die that had multiple punching of a mint mark would strike coins with multiple impressions of the same mint mark letter. Such specimens are called re-punched mint marks, or RPM’s. In the Coin Collecting Hobby these Re-punched Mint Marks (RPM) are very collectible.
When the Mint started using mint marks (letters) in the early 1800’s to identify the various branch mints at which coins were being struck, the mint mark was hand punched into the working dies that would be striking the coins. It was the last portion of the design to be placed on the die. These mint mark letters are as follows: D for Denver, S for San Francisco, CC for Carson City, O for New Orleans, P for Philadelphia, and W for West Point.
A Mint engraver would take a thin steel rod (punch) that had the mint mark engraved on one end and hold it in place on the working die where the mint mark was to be applied. Using a mallet he tapped an impression of the mint mark into the die.
In most cases it was necessary to strike the punch more than once with the mallet in order to leave a satisfactory impression of the mint mark in the die. When the multiple mint mark impressions are from the same mint mark (a D punched over a D, or an S punched over an S), the variety is known as a Repunched Mint Mark (RPM) variety.
Re-punched mint mark terminology
The reference of D/D is used to refer to a “D punched over a D.” Likewise, S/S is used to refer to an “S punched over an S.” So in today’s modern coinage, the most affected mint marks with RPM’s would be the D (Denver) and S (San Francisco) mint marks as they are the mint marks most familar to the collectors.
When people describe mint mark punches, a direction may be implied the D/D or S/S mint marks, such as D/D North or D/D West. When a direction of an RPM is given, that direction refers to the direction of the weaker mint mark punches. The weaker mint mark punches were the first to be punched into the working die and did not penetrate as deeply in the working die. Eventually a stronger primary punch would be the deepest impression in the working die, completing the addition of the mint mark symbol and creation of the RPM on that working die.