Great news John and appreciate the additional images, which I took the liberty of harvesting. On further review
the barrels are Plain or Common Twist which would be in keeping with Kirk's estimated DOM
True Stub Twist barrels were uncommon after the 1880s
John Henry Walsh (Editor of “The Field”),
The Modern Sportsman's Gun and Rifle: Including Game and Wildfowl Guns, Sporting and Match Rifles, and Revolvers, 1882
http://books.google.com/books?id=OLwUAAAAYAAJAt present the selection is from three kinds, viz., first, Damascus; second, laminated steel; and third, plain steel.
The chief difficulty in the present day is to obtain iron of sufficiently good quality to mix with the steel, whether for Damascus or laminated barrels. Formerly horseshoe stub nails were alone thought good enough; but of late years these have fallen off in quality, and are also insufficient for the supply of the increased demand for shot guns since the passing of the present game law.
How in the world could 100 year old "time bomb" Twist barrels survive 1200 BAR Proof?!? 
12g 76 mm = 3” LEAD “High performance/Superior Proof”
Service 1050 BAR = 15,229 psi
Maximum statistical individual pressure 1200 BAR = 17,405 psiMagnum proof 1320 BAR = 19,145 psi
