I picked this item up about 2 years back for about 3 dollars in a Texas store. It looks WWII but not sure. Also something about the camo makes it a bit confusing. It says US and Army Services Gas Mask below it.
Yup. Possibly done in theater, possibly done post war (hunter, kid playing "army", etc.) and it's just filtered back into the surplus shop. Hard to say.
If you are lucky, it could be Airborne used, such as 509th PIR. They painted their uniforms in camouflage similiar to what is on your bag. But many reenactors have done it for years so it is hard to say if the camo paint is original or not. If I were you, I would post it on usmilitariaforum.com. You will find answer there right away.
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Best quote of generation by Mr.B:: dude #$%^&* ww2 navy LMAO.