I have been looking for this information as I have it somewhere in the house. From what I have been able to determine, the letter code represents the year and the number is either the inspector or the inspection location. The letter code is alphabetical but there was a change from the Great War to the Second War. I am trying to find the primary source information I have on this.
My Uncle before joining the Army in WWII worked at Gibbard Furniture Factory in Napanee. One of the items the company manufactured was wooden ammunition boxes and he has told me that there was a government inspector who worked at the factory checking the boxes as they went out. So perhaps each inspector or a factory had a code.