Which military branch's retail store system was the first to officially use the term 'Post Exchange' (PX), establishing the model for other branches?

The Post Exchange (PX) system has been a crucial part of military life for generations, providing soldiers and their families with retail essentials and a touch of home while deployed. This retail network has evolved from simple sutlers' tents during the Civil War to modern complex operations worth billions. Test your knowledge about this unique military retail institution that has supported troop morale and welfare throughout American military history.

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U.S. Army
U.S. Navy (later called Navy Exchange or NEX)
U.S. Marine Corps
U.S. Air Force (originally part of the Army Air Corps)