Banc D'Epreuves Des Armes a Feu De Liege (Proof House for Firearms of Liege) report from 1906
https://books.google.com/books?id=5fxGAQAAMAAJ&pg=PA53&lpg First Obligatory Proof Load (
Provisional Proof) for Double-Barreled Breech-Loading Sporting Guns -
breech plugged tubesThe bore is ascertained by means of cylindrical steel rods, graduated to tenths of a millimeter, and other apparatus.
The size of the barrel at its muzzle is taken, if it is the same size along the whole of its length, whereas if the barrel is choke-bored the dimensions are taken at the muzzle and also at 22 centims. from the breach. The size of the bore is then stamped with a steel punch on each barrel, those which are choke-bored being
marked with their diameter at the muzzle, and also with the diameter of the barrel 22 centims, from the breech, as has been explained.
The size of the bore thus marked on the barrels by the controllers indicates to the men charging the barrels for testing the quantity of powder and lead to be used in order to create a corresponding pressure on explosion to each barrel tested.
In addition, the number indicating the size of the barrel at its first test effectually prevents the maker from altering the size of the bore after testing, such alteration being directly contrary to Section 15 of the law.
Second Obligatory Proof - joined barrels, fine bored and finished
The barrels on being received for the second proof are again measured for the dimensions of bore, and an infinitesimal alteration in diameter of 1/10 millim. (0.003937 inch) is permitted between the measurements impressed on the barrel at its first test, and those when it is accepted for second proof. This alteration of
1/10 millim. in the size of the bore is allowed for grinding and polishing the interior of the barrel. If the difference between the first proof figures and the measurements taken before the second proof exceeds 1/10 millim, the arm is refused by the controllers of the proof house.
Third Obligatory Proof for double breech loading sporting weapons; finished barrels attached to finished action.
Yes, the tubes were sourced in Belgium and carry the First Obligatory (Provisional) Proofmarks. I deleted the confusing Third Obligatory Proof stuff in the previous post.