This proof of the bbls only was also once commonly done in England also. There were two sets of marks, one for prelimiary or provisional proof which was carried out at an early stage on the bbls only. There was then a mark for Definitive or Final proof which was for the assembled bbls & action. At a fairly early date a provision was made that an assembled gun could be submitted for a definitive proof which did not have the provisional one for the bbls. It was given a different mark than the regular definitive one & proofed with a heavier load than ordinarly, but not as heavy as the provisional one. My I Hollis muzzleloader is marked with this combined proof mark denoting it only received the one proof.
The provisional proof was I believe primarily set up for the purpose of catching bbls having faulty welds, prior to a lot of work being bestowed upon them only to have them fail proof later. The bore was unfinished & a plug installed in the breech for the proof, which was accomplished with a very heavy charge.
I suspect a lot of this dual proofing was phased out over time after bbl production became virtually universal of steel construction. While flaws in steel bbls are not unheard of, they are not doubt less common than were faulty welds in composite bbls.
