"The Broad Arrow", and "The Lee-Enfield Story by Ian Skennerton, afford many specifics of manufacturers' and unit codes and proof marks, and of rifles of Enfield origin respectively. Akin to the longstanding hallmarking system for British silverware, in which letter codes relate to years of manufacture or importation, is an equivalent employed by the British Proof Houses.
The problem here is that, unlike silver hallmarking, the Proof House codes were only introduced in 1921 and have been only intermittently applied since then, almost on the whim of the Proof Master incumbent at any particular time. production was proved at Birmingham and the marks should therefore comply with these series.
The mark was modified to that shown in Figure II, with D to the left representing 1953, and the B to the right identifying the Birmingham Proof House. Thus the year codes have hitherto been understood to be 1950 - A; 1951 - B; 1952 - C; 1953 - D; 1954 - E; 1955 - F; 1956 - G; 1957 - H; 1958 - J; and so on through to 1974 - Z; ................
From 1975 a further modification was made to the mark, as in Figure III, with another adjustment soon after to Figure IV.The system ceased to be used during 1941, since there was practically no civilian firearm production for the next five or six years, and, with war-time production levels reaching unprecedented proportions, almost all military proofing was effected within the various manufacturing facilities by Government inspectors. However, such date codes as there are are still useful in dating the many firearms manufactured between the First and Second World Wars, including much output from the Birmingham Small Arms Company ( see also BSA Rifles), as indeed is true post 1952 for those rifles more recently falling into the classic class. These marks are also not to be confused with the crossed flags stamp of the miltary proof markings, which may carry similar letter codes identifying the country and/or place of inspection.