Filled Die Error?

I've seen this same error on other 1974 South Korean 100 Won coins: The "7" in the date seems to be missing much of its relief profile.

Is this what is called a "filled die?"

 

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Yes. They call it a Grease Filled Die but it could be any type of debris.

 

Grease. That makes sense. I have a "struck through grease" error, too:

 

"Grease-filled die" and "struck through grease" are the same thing. The grease becomes compacted with other debris, and this is what gives it strength.

Chris