Daryl, I agree,I am of the opinion that mossyrock,s gun has never been modified.
I have seen guns fitted with origional 10&12.G barrels, but in every case the action was built as 10 G. with the 12.G barrels made to fit the 10.G action.[ still a common practice on modern guns in smaller gauge sizes.]
The 1868 rules of proof,for London and Birmingham show that every gun was to be marked with its nominal gauge. However,the rules show that for example,a 10.G bore, stamped 10, was to be divided into three specific diameters; .775 -.770-.760 [ But not marked on the gun] and that for each measured size, a different proof charge was to be used.In 1888 new rules of proof came into force. This I believe is the date when the rules of proof specified the more commonly encountered numbering system, For example 10.G bore size was restated in the rules as follows.
11=.751 11/1=.763 10=.775 10/1=.784 10/2=.793 inches.
All guns with the above series of marks were chambered for 10.g shells, chamber lengths being at the discretion of the gun maker.
All of the above bore sizes were subjected to exactly the same proof test whether tested in London or Birmingham
