1940-S RPM And"9"

Die Deterioration - basically die wear starts chipping at the outside edges of the digits on the die and leaves behind a ridge on the coins. Usually some parts of the design are affected more than others. Commonly this is true the closer to the edge of the design you go, but sometimes (like on your coin) this is not the case.

Some small part of the clue on this coin is that the same effect happened on the date AND the mintmark. Because the mintmark was punched into this die by hand after the design was complete on the die, there's no way that the same thing could affect both the date and the mintmark prior to the die being hung on the press to make coins. Anything that affects both HAD to be the result of minting coins - thus die wear.

ALL die varieties - doubled dies, repunched dates, repunched mintmarks, over dates, over mintmarks - happen to the die BEFORE the die strikes its first coin.

This is one of the MAIN things that makes die varities a different animal altogether from errors. Die varieties are NOT errors and errors are NOT die varieties. They are completely separate from one another, and you will find that collectors are separate on them as well. I know a number of people who collect one or the other but not both. I do die varieties. I know Mike Diamond generally sticks with errors only...for instance.