Me thinks you need to have that gun looked over by someone who knows British guns. Bore diameter, wall thickness and extent of pitting will all play into the final answer as the the guns overall condition. opening the bores to remove pitting will add choke, but on a Purdey there is usually not much you can open due to how finely the barrels were originally struck. Bore work by definition takes a gun out of proof. The red stain would be appropriate on that gun, all of my Brit guns with original finish from that period have it.
Steve

http://www.bertramandco.com/
Booking African hunts, firearms import services

I miss Monkey Jim.