Fazakerley Serial Number

I just got a sportered Fazakerley No4 marked 11/43. The barrel is already cut behind bayonet lugs and the wood is cut down so I was going to use it to build a so called "Tanker" by shortening the barrel and adding to the forestock. I would like to check out some of the markings first. serial number is faintly electro-penciled on the wrist and stamped clearly below that, number is also very faintly marked on top of buttstock inside the butt socket. There is a T stamped on buttstock behind trigger guard. Serial is JB245xx. Just curious, I haven't seen a JB prefix in serial number before and wondering why the T is on the butstock.