I am not 100% sure, but I think it has something to do with the age or series. Like a 1 would be a 1,000 year generation. I am probably wrong, but I
got that way of thinking from a post on ATS from earlier today.
The P is a reference to the way that you are paid (PAYE).
PAYE-45
PAYE-60
The numbers are just a code reference for that particular form. I mean, if you issued a P, that would not mean much when we have so much government
jargon.