Interesting. The only chamber markings that were usually on the barrels water tables were for the Chamber 3 Inches on Longrange Wildfowl guns and some Trap guns. I also have a 20 ga. Ideal Grade with Chamber 2 3/4 Inches, but we think it was put on there. Most 20 ga. guns up until close to the 1940's were 2 1/2" chambers. I would maybe venture to say that your gun because of being in the UK, and the laws there, that it might have been bored to 2 3/4" chambers. Does it have proof marks? Those proof marks if there would be from the UK not from here as guns of that era were not proof marked. If you could post some close up pictures of markings. Unless checked out for chamber length, I personally would not shoot 2 3/4" loads through it. I would try to stay with loads under 8,000 psi. I'm sure there is a way to convert that to your loads, and yes you could shoot heavier shot staying in the 8,000 psi range.