Just adding this for reference , but the letter codes for WWII British arrow marked items is as follows;
R=1939 Q=1940 P=1941 O=1942 N=1943 M=1944 L=1945 though Z can also equal 1945
The post war sequence is not known, but it should be possible to work it out if enough dated and arrow marked items were available for study. During WWI it appears that they used a code sequence of letters based on the issuing depots, which has been posted on the Great war forum. I don't think any work has yet been done on the interwar years, but I think they put the date below the arrow in most cases.
Shown is a double marked item of headgear with an arrow marked and dated label and a Letter code marked broad arrow stamp.