No.5 ROF (F) question

[Beerhunter]

I have a No.5 that is all matching however, the rear sight is of the correct type 800yd variety but is marked with a B on the major parts would that make it a BSA item?

Only two factories made No.5s, ROF Fazackerley and BSA. I suspect that when doing repairs to rifles in the field, armourers did not try to match parts from the 'correct' factory.