Good find Scooby ......
According to Q. David Bowers there are an estimated 400,000 of these 1938 D/S minted .... with at least six varieties known of the over-punched MM.
Mr. Bowers goes on to say ..... paraphrase here ....
One theory of how they came to be is that as dies were being prepared for the 1938 year someone realized coins would be minted in Denver and not San Francisco ....... several (6?) of the reverse dies were sent from SF to Denver where the MM was over-punched with a "D".
I believe you have one of these six varieties ...... great for you.
You can read more about this coin in Mr. Bowers book "Buffalo and
Jefferson nickels"
David
Take a look at my other hobby ...
http://www.jk-dk.art