Vic Venters book "Gun Craft" contains interesting information on the rules of proof. When the Brits went from the 1925 rules (the first time that chamber length became a required proofmark), there was some confusion. Venters, in an interview with former Birmingham Proofmaster Roger Lees clarified it in a couple of sentences: "The proof loads now set down in writing (in the 1954 Rules) are almost in every instance the proof loading in use under the1925 rules. In general it may be said that under the new Rules of Proof, no arm will receive a more severe proof than hitherto."
Venters goes on to explain that the major changes which took place when Britain joined the CIP were that the new marks use the metric system and are now proof pressures as measured by transducers, not service load pressures. (Gun Craft, p.206._)
