Like most machines, it is like the lubricant that acts as a lubricant to facilitate the journey of an engine to reduce friction. In this case, the grease flow to the surface of the coin die and eventually plugging up the number '20 ',' S 'and' E '. Then, when the coin is struck to the die surface of the blank planchet, the number '20 ',' S 'and' E ' were not fully transferred to the surface of the coin. This type of error is called struck through grease filled die or filled die error.
