Military Postal Express Service duplex handstamp
The Chambers shop at Lodge, Virginia, produced this postmarking device around 1917. This flange-style device can be distinguished from similar postmarkers with screw-on heads made after 1931 by the 'year' type being positioned in a slot between the canceling bars and the postmark dial.
The Military Postal Express Service of the American Expeditionary Forces existed during World War I to deliver and dispatch solders' mail. The number between the canceling bars indicated the unit number. Mail was addressed to these army post office numbers to conceal the location of military personnel while still assuring that mail reached them.