Thank you Alan,
I thought that might be the case.
The diameter of the rails is the same for 4x5, 5x7 and 8x10 and interchange between the Norma, F series and P series. The first ones were silver and the later ones black. Just look for a silver one if you care about it matching your P.
And I think the extension rail has red tips while the "base" rail has black. I got black and it measures 12". I'm going to look for a silver, red tipped extension rail,