Gray Ghost Moderator
IIRR the "double circle" has a different meaning. More like an accepted sample. But I need to cross check that.
The attached image is from the Marineverordnungsblatt of 1939. Sorry for the poor scan and hard to make out the details but this is the best image I could find. As Wolfgang stated it is apparently an acceptance stamp for parts and materials. Should not be confused with the Eagle > M stamp which is a property marking.