The AM is part of the serial number. As in 'AM12345'
British No.4 rifles have five numbers, usually after one or two letter prefixes. The same letter prefix(es) were used by Maltby, Fazakerley & BSA Shirley, A to Z then AA, AB to AZ, then BA to BZ, CA to CZ &c. Maltby rifle serial numbers commence with a number '1', Fazakerley with a '2' and Shirley with a '3', e.g. 1###x for Maltby, 2###x for Fazakerley and for Shirley, 3###x, after the letter prefix.
Long Branch (Canada) serial numbers incorporate an 'L' in the serial number while US Savage numbers include an 'C' in a similar relative position amongst the numbers. Both of these No.4 rifle series commenced with 0L1 and 0C1 respectively.