That's correct Bar. Never delete the old markings, always bar through. Looks like the engraver at Fazakerley was particularly enthusiastic on that particular day as he's done a decent job, a bit of depth to the engraving and a new or sharp cutter by the looks of it. As opposed to the usual #scratchy pen' type of etching we usually see from Fazakerley.
The P prefix is a regular feature of Fazakerley production and sometimes features in their 'lost' number re-numbering system such as a BSA one that I know of, FTR'd at Fazakerley now numbered 53P 10385