1964 D Nickel RPM/Omm #3?

When you look closer at the mint mark area, you will note not just the reduction, n
But metal movement on the mint mark area:

Note how the direction moves the MM? A RPM is not metal movement, from the mint mark incorrectly punched into the die. The difference? The die striking the coin with the RPM on the de. MD causes damage to a struck device after the strike happened. One is a die variety, the other is machine damage to the coin.