Brass hanger suggests WWI to me. Still, that's not 100%. The best thing would be to post photos of the front, back, flap open, inside and a close up just below the hanger (to make sure the marking isn't extremely faint and/or worn - sometimes photos show what eyes don't see) to the forum and let the experts chime in. At least they will be able to verify it is USGI and not a repro. That in itself would be information.