Means that at one time or another it went through a British proof house and was tested there. They report pressure in long tons (2240 lbs) per square inch. (Actually, Gough Thomas compared pressure readings by US and British methods and says it converts closer if you take the ton as 2800 pounds; due to differences in design of pressure test equipment.)
May have been a surplus sale, back when a free Englishman might own a pistol. If it had been for military issue, it would have been marked "Not English make."
