CJF, The process that the gun in question went through was something that was not that uncommon with heavy wildfowling guns that were originally built for longer chambered thin brass black powder cartridges which appeared around the 1890's in Britain.As time progressed and nitro powders and longer paper cases became the norm,the process of chamber lengthening,rechambering,reboring,and re-proofing,assuming that the actions and barrels could handle the new loads,was done.It is always interesting to try and dissect the process that the gun went through from the array of proof marks.I have a W P Jones 10 bore HG with heavy 34" steel barrels and full and full choke,that went through a similar process and from the proof marks it looks to have evolved from an over bored black powder 12 bore of indeterminate chamber length,to a 3" nitro 12 and finally a 3 1/2" magnum 10.The gun weighs an even 10 lbs. and because of the heavy steel barrels it balances almost 3" ahead of the hinge pin.As mentioned earlier,if only these guns could talk...
A couple pics of the proofmarks
