Oh dear.
I have to disagree with a few people here. I have been to the Wilkinson Sword Factory in London and seen the British Proof Testing machine for swords. A full explanation of the test is mentioned in the book
'British Military Swords' by John Wilkinson-Latham. The reason for proof testing came about as during the Crimea war, swords were found to break or bend in battle. This was termed by the government as the 'Crimean Sword Scandal'. The Government decided something must be done to guarantee the performance of a British 'working' sword. It has to be said that German Swords made in Solingen were found to be far superior. Imagine a cavalry soldier on his horse stabbing at a foot soldier to find that the blade bent at 90 degrees instead of penetrating! He is stuck on his horse with nothing to fight with. If he was lucky, the tip broke of completely and are least he had something that was a bit rough and sharp! The Proof Test was devised. First the blade is put into the machine and it is subjected to a load for a period of time. This load causes the sword to bend dramatically. The sword must spring back to true with no deivation. Another test is to apply a drop hammer blow, ( a certain weight from a certain height) to the back of the sword, with the blade edge on a hardened steel face. The back of the blade must not bend, and the edge must not chip. There are other tests too, but my book is in storage. After passing the tests, the blade is polished and finish sharpened. A gold pellet was inserted into a milled depression in the blade and stamped with a symbol. IIRC Wilkinson Sword is a star. I have one of these just like this one.
I have 5 British proofed swords and one light cavalry pattern sabre which has been bent and straightened (rather badly). One test I do when buying a military sword is to 'wobble it'. Put the tip on the floor and press down and to the side, lift the sword and see if it wobbles. I once purchased an interesting naval sword which looked good as far as the sharkskin bound hilt with lions head pommel looked, but the blade looked a bit 'dull'. I showed it to my collector friend and he did the wobble test and the blade bent! He explained I had purchased a proper hilted sword, but with a replacement mild steel blade! He was mortified he had bent my blade, I was mortified I was so stupid. It went to auction before I moved to Canada. Now if only I had noticed, it didn't have this proof mark, as this is the pattern sword ....
There is only one British Proof Testing machine for blades, it lived at Wilkinson Sword and every single military blade from any manufacturer in the UK were tested on it. I was quite in awe to see it as I had only just read the book!
'Still in Proof' on a shotgun means that the bore measured 8" from the breech is still within the tolerances at manufacture. My shotgun failed and was unsaleable in the UK. It was a great shame, as it was a 21st birthday present from a guy I was working for in 1974 and it was his fathers. The gun was a 12 bore sidelock with browed damascus barrels made by Brown Brothers of Birmingham (where I was born) and had a silver oval in the stock signifying it was superior quality.