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1980-D RPM?
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7 posts in this topic

Rick,

When you see a well defined “shelf” like is present here it is usually indicative of machine/mechanical/strike doubling (all the same thing). 
 

Some good material for learning the different types of doubling, and there are several, here is a good place to start. Understand that only very specific types of doubling are of interest as the rest are common occurrences in the manufacturing process. 
 

Machine Doubling:

http://www.error-ref.com/machine-doubling/
 

Doubled Die

https://www.error-ref.com/?s=Doubled+die

Die Erosion

https://www.error-ref.com/?s=Die+erosion

 

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