Are they raised to a fingertip touch? Could be die pitting due to decarburisation of the die steel. It is seen primarily in the fields along with other signs of die wear. Alternatively could be either contact with a corrosive material or large strike through of grease/debris. Out of ideas after these
Appears to have been exposed to something acidic. Maybe a car cup holder coin? Coke will do funny things to metal. LOL