I just mentioned in a thread a week ago an BSA 1994 No.4 Mk1 which was FTR'd into a Mk1/2 in 1955 at Fazackerly and WAS reserialed with an A 12XX number on receiver, bolt and barrel. Was the serial change because it was effectively classed as a new rifle, having been changed from Mk1 into Mk 1/2?