1. Proof. Requiring a gun to be currently "in proof" applies ONLY to weapons sold, displayed for sale or let on hire. It does NOT apply where a weapon is GIVEN.
2. Current requirements AFAIR now require barrels be struck off when a gun is submitted for re-proof. So that will then involve a re-black of the barrels when it comes back.
3. Proof is, in some ways, a nonsense. All bores of a cylinder wear. The bores of your barrel are no different to the bores of the engine of your car.
So that the bores have enlarged by 10 thousandths of an inch does not per se make the gun dangerous. As long as the rest is sound and good.
As long as the WALL thickness of the barrels is adequate the gun will, even though not "in proof" be perfectly and totally safe. As long as the rest is sound a good.
That rest being sound and good is so important I've repeated it twice!
As an extreme example the barrels of a Greener GP are massively thick. You could enlarge the bore by ten, fifteen, twenty thou and it would STILL be hugely over engineered.
Yet my the nonsense of the Proof Act it'd be no longer "in proof". I see thinner scaffold tubes than barrels on Greener GP guns!
4. Proof marks changed over the years. The gun may have earlier era marks than .719". Here's the equivalents.
12/1 = .740"; 12 = .729"; 13/1 = .719"; 13 = .710". Current proof marks, even in UK will be Metric.
Hope it helps.
5. Ignore this 10 thou twaddle of the Proof "regime". Check the WALL thickness and overall barrel condition for, if the barrels are sound, on the face, the ribs well in place, and the firing pins correct..it's good to use and safe to use.
Any good GUNSMITH (not any Tom or Dick with an RFD) will advise both if the gun is "in proof" and, that notwithstanding, if it is, irrespective of its proof status...if it is SAFE.