Fazackerley No5 production started with a pre-production run of around 1000 rifles with a FE prefix, actual production of the No5 started with no prefix 1 to 9999 (no zeros) then A1 to A9999 then B1 to B9999 & so on thought the alphabet (a couple of letters were left out those being I & J) then AA1 to AA9999 though to at least the AC prefix, you can have 1,2,3,or 4 numbers in a No5 serial number
Fazackerley No5 production is more like 250,000 & not the often quoted number of 169,807 as that number is up to serial number S8823