1944 National Geographic Map - Germany and Its Approaches

I was given this map as a gift.  It was part of my wife's great uncle's WWII belongings.  He passed away in 1994, and this was given to my mother-in-law, along with a belt buckle bearing "Sixth Armored Division" and the respective logo on it.  From what I know, he fought in the Battle of the Bulge with the United States army.  The map was originally printed in National Geographic, but this particular map was printed with occupational zones on it for the military.  This particular map has the description, "This edition for use by War and Navy Department Agencies only - Not for sale or distribution."  I am not interested in selling this map as I am just now getting started with collecting WWII-related militaria.  What I am interested in is the story behind this map and a potential value it may have.  Thank you in advance for any help that can be provided.

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