Very true, but Tons psi (Birmingham proof) or "Tons" (London proof) markings are under the Rules of 1954. If the gun is between the wars, then the chamber "should" be marked "2 1/2", 2 3/4" etc" and not in metric mm's.
Greg
Not quite, Greg. 2 1/2 or 2 3/4 would not have been marked under the proof rules of 1904, which remained in force until 1925. On guns proofed in 1925 or later, you will find chamber length. That's a quick and easy way to distinguish a gun with pre-1925 proof from one with post-1925 proof. Otherwise, those two sets of proofmarks are pretty similar.
Rocketman's caution about 2 3/4" shells is also spot on. And another thing to be wary of would be guns which have been lengthened to 2 3/4" in the States but not submitted for reproof. You'll find quite a few of those floating around on the market in this country. Too many, in fact.
