I've recently aquired an officers cap of the devonshire regiment with stamps inside 7 arrow mark and 18 underneath. I know the letter designations for ww2 (R=1939, Q=1940 ect...) but dont know the stamps for ww1 or interwar. Can anyone tell me the year of manufacture for this cap please? An arrow stamp with 7 above and 18 below. The cap itself has a narrow chinstrap (like ww1 caps) 4 air vent gromets on the rear and looks to be ww1 or interwar.