Kevin,
To the best of my knowledge I believe the overprints are accounted for in the 14 known Brown Old Mills because they also have Brown Old Mill backs, but have the additional factory overprint.
Below are scans of the Greminger single overprint contained in the 2010 REA auction:
Greminger OverPrint.jpg
Below is back scan of a double overprint that I found in a book about baseball cards:
Double OverPrint.jpg
Either Jim or Tim can verify that they are accounted for in the Brown Old Mill population.
Jerry