The info I have on the "tween" sizes came from A Baron Englehardt. According to him under the proof law of 1887, the ord "Choke" replaced the former "Not For Ball" & the B & M marks for bore & muzzle. The mark in the diamond of chamber gauge over c or LC for long chamber were also added as was the Tween gauges sizes for the bore with 3 divisions on gauges of 4-10 inclusive & 2 divisions on gauges 11-17 inclusive.
I have a
1963-65 W & C Scott & Sons pinfire built under Mathew's patent. The top lever is shorter & set further to the rear of the breech than a typical Scott Spindle. This gun also has a single underbolt. I will have to look & see, but as I recall my gun has a reference to that patent stamped on it.
My understanding is that guns built prior to 1868 were gauged at the muzzle. After 1868 if the gun was choke bored this had to be stated by the maker & then it was gauged from both the breech & muzzle with gauge marked as for example 12B/14M Not For Ball. Cylinder bore guns were I believe still often gauged from the muzzle prior to1887.
From everything, I have ever read on the subject prior to 1887 the proof house only listed gauges by whole sizes, ie 8, 9, 10, 11 12 & Etc. down through 50 gauge 0.453"). From 50 gauge down in hose early days, they began at .450" (51.05 gauge) & went down in.010" increments with the gauge number rounded to two decimal places. What we today call a .300 caliber was then listed as a 172.28 gauge.
