So as an interesting aside to this discussion I thought I would mention a lovely Purdey that recently came through the shop. In very high condition and built as a 2&1/2" gamegun that was ordered in the late 20's, never collected by the chap who ordered it and remained in stock until the the US Naval attaché was dispatched to London in the early '40's who then purchased the gun. The gun had its chambers lengthened to 2&5/8" and the proof marks defaced but the gun was NOT re-proofed. Purdey verified that the chambers had been lengthened before the sale. I do not believe a 2&5/8" proof length was ever a standard. While I generally agree that lengthening chambers on British guns takes them out of proof, I am a bit uncertain as to how 2&5/8" chambers fit in the equation.