When you turn the coin so that the flow lines reflect at you and the flow lines are gone in the area affected (missing letters) with an irregular hard line around it, you can ALWAYS count on
Grease Filled Die as the cause.
The only time you will see something similar is on the opposite side of a
Cud, where the planchet doesn't fill the die because there is no opposing die to force the issue. Thing is, 99.999% of people would notice the
Cud LONG before they would notice the missing details on the other side. So when I see a pic of missing details WITHOUT a pic of a nice
Cud on the other side...the result is obvious to me.