'Clochandighter' is listening loud and clear.
Firstly, the pillar at Rottington is S1509 to me, not TP.... whatever. I say this randomly a few times a year in my responses. TP.... is a URL related number on this excellent website but bears no relevance anywhere else. S1509 is the number physically on that pillar.
Back to the question in hand. This Primary station is PP1 or PP001 as far as the Ordnance Survey are concerned. That was a decision taken in 1936, long before I was born, and even before my mother was born, Therefore there is a limit to what I can explain on why a Cumbrian coastal pillar became designated as such.
Seriously, the designation PP1 must have been allocated in a drawing office c1936, possibly after the pillar was built and as surveying was about to begin. As you correctly state, the first OSBG36 sightings were taken from S1534 (Cold Ashby), despite that having been allocated the OSGB36 handle PP76.
From the limited information I have, the first pillars completed were 2949 (Easington) and S1500 (Martinsell) late in 1935. Although fairly early in the FB number sequence, neither were first to be observed.
Back at S1509 - Rottington, it was an honour to bag PP1, hence my term 'Mecca' despite it not (quite) being physically the oldest pillar, nor it being the first observed - blame the 1930s drawing office.
As for guides, there is a Word companion for Pillars16.xls but that only contains a brief history and an explanation of the codes used. PillarHistory.xls is the expanded version and is still ongoing so the guide has yet to be created. That will happen one day.
Hope this helps,