Rabbit - I have an early 60s 203 12 bore that was originally sold through a London shop and bears London proof marks of 31/4 tons. I too struggled through various posts discussing conversions between proof systems and the difference between service and proof pressures. I am likely being over-conservative, but I also concluded that 10,000 psi was an appropriate limit and decided not to exceed 11/8 oz, 31/4 DRE. My gun weighs 6.75lbs so shooting loads in excess of that might get a bit uncomfortable in any event.
