BSA commercial FTR's of the 50's serially numbered their rifles again, starting from A1. Don't ask why 'cos I don't know and it certainly wouldn't have happened with UKicon Military FTR's as we would renumber using a set ZZ prefix or SA61Axxxxx type number So yours is an unknown manufactured No4 upgraded to Mk1/2 by BSA in 1955 and re-numbered to boot

BSA were a private company and this was an easy way to make money. Buy zillions of rifles cheap from the MoD, rebuild them and sell them on to nations wanting to re-equip with No4's. The Sierra Leone and Ghana Armies were BIG buyers of these rifles

Spike DD can elaborate more on these BSA commercial dealings but there are exceptions to the renumbering. For example, by nation returning rifles for FTR simply wants the same rifles back, FTR'd