Pretty certain was not a listed optional extra from Princess and would have been fabricated post purchase by first or subsequent owners.
Last P35 (1992 on) had transom door as standard and earlier ones you could ask (pay) to have it fitted.
Many side boarding ladders very probably lying on river beds somewhere by now.
We recently invited a local diving club to visit our moorings to retrieve a number of items which hopefully were lying near where they "fell" in, this included a couple of boarding ladders.
It does help if you actually fasten the supplied ladder securing rope to the boat.
Reckon going to mean a quick measure up and visit to your local stainless fabricator.