Brit L1A1s use a two letter code followed by a two number code (year of manufacture), followed by the actual serial number. For example UB 59 A123456 (UB means made by BSA, 59 means made in 1959 and the rest is the actual serial number). As mentioned, Aussie rifles are AD followed by the year and a five digit serial number. For example AD7501368 would have been manufactured in 1975.