Agree with ellenbr, the word choke was first used in 1887 as was
12/C enclosed in a diamond. The latter mark was used on guns with chambers upto 3 inches. steel barrels were used on english shotguns as early as 1864 these barrels were produced by acold drawn process. by 1874 whitworth fluid compressed steel barrels were available.
Formal nitroproof marks were not used untill 1896.

Roy Hebbes