The Parker factory in England was in Newhaven, N for Newhaven, just to round this out
Other Parker UK nibs may have different numbers
4 - Parker 17
5 - Slimfold
10 - Junior (& Victory V)
(15 - Victory)
25 - Demi and standard
35 - Senior
50 - Maxima
N - Newhaven manufacture
This is just the most basic of information, there are some dated nibs and some odd numbering that doesnt fit the pattern above, Parker for example would replace a damaged nib with anything that would work, for example I have a Parker Vacumatic that had a Duofold nib fitted by Parker and another Parker AF that has a nib without any markings other than Parker.
One minute you are driving down the road, every window open, singing along to 'I dont want to miss a thing' at top volume, the next you are driving and turning down the volume on the radio to help you see better.