Original Purdey record says
Purdey No. 15319
Completed - 1895 for The Duke of Leuchtenburg
Calibre - .500 back powder rifle
Barrel length - 27" Whitworth steel
Stock length - 14 11/16" with Silver heel pad
Weight - 9 lb 12 1/2 oz
Action with 3rd grip and clip sides
Lever over guard opening

The top rib doesn't look replaced to me, but sights, yea, removed. The one of hooks looks terrible, somebody wanted to make barrels on face and hammered this hook.
To restore this gun professionally would cost $$$.

Geno.