From the width of the action and the original proof marks it appears to have been built as a Black Powder Express.

The added marks do not appear to be any sort of official proof house marking.

Assuming the bores are in good condition and it locks up snugly I would only use it with Black Powder loads or “Nitro for Black” loads which are designed not to exceed black powder pressures.

Do NOT confuse .450 Nitro Express with .450 Nitro for Black, even if they both seem to fit the chamber . It is like the difference between using a heavy steel hammer and a wooden mallet.

Try to get a copy of Graham Wright’s Book “Shooting the British Double Rifle”