This is an example of the M1917 Gas Mask Bag, as used by American troops during the First World War. Named on the front flap to W. W. Stover. There is some other writing front and rear, but I can't make it out, even with a blacklight. Bag shows general wear and tear commensurate with age, but no significant rips or damage.
The Great War enlistment records show three W. W. Stover's having signed up.