Whether it is "Nominal" 2" or 2 3/4" actually has little to do with its age or whether it has only black or smokeless proof as well. Many nominal 2" British guns, in reality, had either 2 9/16 (65mm) or 2 5/8" (67mm) chambers. What actually determined the chamber length was the gun's intended purpose. Game Guns normally had the short chambers, with Pigeon or waterfowl having longer chambers, up to as long as 3" (82.5mm). I recall Nash Buckingham mentioning using a Greener hammer gun of his father's back in the 1880s which had 3" chambers.
Larry was correct that the chamber length was not marked at that point. Measuring it will tell you what it has "Now". If the gun weighs in less than
7 lbs the odds are it started life with the shorter chambers even if they are longer now. I don't recall for certain when the 2" chamber was introduced, but don't think it goes back far enough to have been used in a gun stamped "Not For Ball".
I highly recommend the 6" flexible scale method. You cannot accurately "Feel" the start of a normal cone, but you can hold the barrels up to a light source, not overly bright though, & looking through the bore measure to the shadow line keeping the scale along the inner wall & where you can see it.
