'Y' Marking on an No.4

I've had a good dig around here and looked at some other websites but i can find anywhere what a 'Y' on the butt socket means. The No.4 is a Mk1/2, its got the 'suncorite' finish and i can only make out the original Mk1 production date as 194- , cant read last digit sadly. Its does have M47c (BSA Shirley?) on the butt socket but read about it the serial number begins with a 2, was the serial number changed when it was FTR'ed at Fazakaley? I'm sure i read somewhere that 2 at the begining has something to do with Fazakaley.

probably need some more pics than the one you have buddy.