Drew, your post pretty much confirms what the former proofmaster said, although he was referring to the 1925 rules and not 1896. He served both before and after the transition from the 1925 rules to the "tons" rules (in 1954), and said that the proof remained the same even though the proofmarks were changed.
Taking one more step, British shell boxes will tell you that standard proof CIP loads are OK in 2 1/2" guns proofed at 3 tons or 850 bar; 2 3/4" shells in guns proofed at 3 1/4 tons or 900 bar. Equating 3 tons to 850 bar and 3 1/4 tons to 900 bar would seem to contradict published transducer pressure equivalences for the "tons" mark, because we know that a gun bearing an 850 bar proof (current CIP standard) equates to a transducer-measured service pressure of 10,730 psi. Something seems out of whack there. However, there are a large number of Webley & Scott post-WWII 700 series guns that were imported to this country with 2 3/4" chambers, 12ga guns marked as 3 1/4 tons. And most buyers likely assumed that since they were made for the US market, that they would handle standard US 2 3/4" ammunition. (I owned a 20ga Model 720 with factory 3" chambers--basically a 702 made for the American market--that was marked 3 1/4 tons. Since it weighed less than 6 pounds, I never had the guts to try it with 3" magnums, and in fact limited factory shells to nothing heavier than an ounce at modest velocity.) Likely not comfortably from a recoil standpoint, if the shot charge were in excess of 1 1/4 oz and/or the velocity in excess of 1300 fps--and especially if both were true. But many have been shot for a long time with lighter and/or slower American loads, and I've not heard of reported disasters as a result. Personally, I too would exercise caution--if for no other reason than excessive recoil. But it is a contradiction in the interpretation of acceptable loads based on proofmarks.
