1970 s DD or md in the date and mm

the date i was thinkinking machine doubling but the s maybe an rpm thanks for any input.

 

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Same doubling as the date. MM were hand punched into the working die before hardening.
If you find a coin that has the same type doubling on the MM as the date then it is no doubt a type of Mechanical doubling.

 

Thank you

 

Notice how it's flat and cuts into the design? True doubling doesn't take away.