As to chambers, I will point out just three of the guns I own.
First is a ca 1893 12ga Lefever G grade damascus with 2 5\8" chambers. Normal for the era so undoubtably original. This gun weighs 8lbs even.
Next a high SN'd 12ga Lefever H grade twist, probbly assembled at Ithaca post 1915. This gun weighs 6 1/2 lbs yet has 2 3/4" chambers. 2 3/4" chambers are "Normal" on late model Lefevers so this one is most likely original as well.
Now the G has quite heavy bbls & the chamber could in fact be extended to 3" & still have thicker walls at the end than the H. I definitely would not recommend re-chambering this old gun to 3" & firiing magnum loads through it, "BUT" in this era Lefever as well as other makers would cut chambers to order. No doubt had that G been ordered with 3" chambers there would have been absolutely no difference made in its bbl configuration.
Last gun is a post 1907 16ga Lefever H grade twist. This gun came to me with 3" chambers, yes you read right, A 3" 16ga. Are these original, well who knows, but Why would it have been re-chambered to 3". At the time it was built 3" chambers could be had to order along with shells to fit. They "Never" became a standard so I highly suspect it was ordered new this way.
It is also noted that many early Lefevers, along with some other makes, came from the factory bored oersize for use with all brass shells.
Does anyone see the can of worms we would be opening up here. None of these early guns were factory marked with either chamber length or bore size.
"WHO" would determine which ones had been modified & needed to be proofed. Bottom line is to ensure all modified guns were proofed every gun sold would have to be subjected to the proof.
"IF" it were not "Mandatory" it would do nothing to protect the "Unknowledgable" Joe Blow on the street that it is suggested to protect, voluntarily wouldn't work. Even then it would still offer no protection to anyone dragging out an old ancestrial short chambered JABC etc & stuffing it with a high powered load.
Best thing to do with this subject is
"KILL IT"
before the wrong people read it & get ideas, "For Our Own Good".
