Starting after WW2 the Brits started to make an ever increasing percentage of their standard guns with 2 3/4" chambers and by the 1970's these were the majority of new guns produced. Why? Don't know, maybe just for standardization in manufacture, foreign demand escalating, domestic demand escalating? By the late 1980's Britain was part of the EU and they were using metric proofs. Because some of the parameters of the metric proof laws were different something had to give and it turned out the be the Brits. The amended EU proof laws required all guns to be proofed with 2 3/4" proof cartridges so submitting a gun for proof that had 2 1/2" chambers caused a few proof failures, especially on reproofing old guns. Then the EU drove the final nail in the 2 1/2" coffin by demanding a double proof with these 2 3/4" proof rounds ( two shots per barrel) so the Brits pretty well abandoned the 2 1/2" chamber, even for reproof of older guns.
Choke was marked in different ways through different updates of the proof rules, starting with the separate size of bore and muzzle, as 12B and 14M being the most common for a choked 12 gauge. This started with the 1875 rules and for many years after this a lot of guns were made without significant choke. Some were subsequently jug choked or were choked as part of a reboring to clean up a pitted barrel. I have run into original cylinder bored guns from as late as 1905, not everybody wanted or believed in choked barrels and some old timers didn't believe in them, sticking to their old ways. Some of these old timers also continued to order an occasional new muzzle loader well into the 20th century although very rarely after WW1.
Oddity - Nitro Proof was optional until the proof rules of 1925 so you find a few guns made up to this time with only black powder proofs which is really confusing on a gun that was apparently made about 1900-1915. Apparently proofing was paid for by the maker who submitted the gun and Nitro Proof cost a few pence extra so sometimes this was omitted on a gun that was to be exported to a country without proof laws such as Canada or USA. I once had a very fine Westley Richards gun made for the owner of the Rogers Sugar Refinery in Vancouver in 1912, 2 3/4" chambers, no Nitro proofs. It didn't make sense at first until this little quirk was explained to me. J
Choke was marked in different ways through different updates of the proof rules, starting with the separate size of bore and muzzle, as 12B and 14M being the most common for a choked 12 gauge. This started with the 1875 rules and for many years after this a lot of guns were made without significant choke. Some were subsequently jug choked or were choked as part of a reboring to clean up a pitted barrel. I have run into original cylinder bored guns from as late as 1905, not everybody wanted or believed in choked barrels and some old timers didn't believe in them, sticking to their old ways. Some of these old timers also continued to order an occasional new muzzle loader well into the 20th century although very rarely after WW1.
Oddity - Nitro Proof was optional until the proof rules of 1925 so you find a few guns made up to this time with only black powder proofs which is really confusing on a gun that was apparently made about 1900-1915. Apparently proofing was paid for by the maker who submitted the gun and Nitro Proof cost a few pence extra so sometimes this was omitted on a gun that was to be exported to a country without proof laws such as Canada or USA. I once had a very fine Westley Richards gun made for the owner of the Rogers Sugar Refinery in Vancouver in 1912, 2 3/4" chambers, no Nitro proofs. It didn't make sense at first until this little quirk was explained to me. J
