Re-cut date or Machine Doubling?

What accounts for the appearance of these numerals in these dates?
These are S. Korean Five-Won coins. This appears to be quite common on several pieces that I and some others have.

Also, what are some of the best resources (books) that address this kind of phenomenon on coins? I'd like to check them out...

 

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Maybe a case of Die Deterioration.

 

Longacre Doubling?

 

Looks similar to a Silver Eagle that I have. I would be interested to know what it is, as well.

 

After having looked this up, "Longacre Doubling" does make sense. Especially if done to improve the life of the die.

 

With no other evidence than this I consider them Machine Doubling. Near as I can tell, both "strokes" fit into the intended dimension of the original stroke. This is a classic sign of MD, and not possible with die doubling.

 

I have nine ASE with MD on the date as well as the sandals and toes. There are two of those that I have no clue as to how they got out.