Looking at your latest proof marks (the 1954 onwards table) BNP denote Birmingham Nitro Proof and NP denotes London Nitro. If you are in the US I would recommend also looking at the chamber length as marked. That defines the case length ( 65mm being 2 1/2" and 70mm being 2 3/4". That means for example a standard game load, these od guns were not built to handle any magnum or hot shells.
I would be surprised if the post 1954 re-proof did not cover Nitro, if it as for B.P. only there would be some mark to indicate that.