Unusual to see only one tapped hole for scope base mounting at the rear of the 404 Parker Hale. Most of the PH rifles I've seen and handled all had tapped holes for two piece bases. I have mounted a lot of scopes in early years using Parker Hale bases and don't recall seeing any for Mauser actions in one piece as opposed to the standard one piece for the SMLE 303 rifles of which I fitted many.

You need some of the original Parker Hale 404 ammunition for your rifle now to complete the set. I still have a little left that came with my Mauser 404. Used some on Aussie buffalo and a few other shots over the years, it was made by Norma and performed superbly on the buffalo.
Hope you can locate a base for your rifle.