Note that beginning in 1868 & continuing through 1925 there are two marks with the word "OR" between them. The mark to the right is the ordinary preliminary/provisional proof, which was applied to the rough barrel tube before much work had been put into them, so if they failed not as much labor was lost.
The mark to the left is the same provisional proof but it was applied after the barrels had been completed & actually at the same time the final/definitive proof was applied. Apparently by 1865 barrel-making had advanced enough not so many barrels were being lost at the provisional stage so the gunmakers were given the option of all the proofs being applied in the finished state, though still in the White. This cut down on their expense as the barrels made only one trip through the proof house instead of two.
My, I Hollis & Son 12 gauge carries this optional proof mark so was made after 1868. It has the bore mark. The only older shotgun I have is a W&C Scott pinfire which dates to ca 1863-65. It carries the standard provisional proof mark & also has the bore marking.
