I assume this marking is on the barrel nocksform - that is, separate from the proof markings closer to the stock?
A Crown/B/# (crown over B over a number) marking was a Government inspector's mark for Enfields inspected for Government purchase/delivery from Birmingham. These Government marks were not normally found on commercial Enfields (that is, those that were sold to North or South), but this may indeed not be that same marking.
In any case, I agree with Tom - the B would mean Birmingham (the source of almost all P53 Enfields with lockplate markings and bore gauge markings as on yours).
Or ... it COULD be a marking that indicated the barrel maker - is there a name on the bottom of the barrel?