Pooch, I'd say 7500 psi is a reasonable upper limit for that gun, once it's back on face.
Re the A&N records, they're great. I paid for everything on an A&N pair I used to own, but that information did not include original chamber length. No big deal in that particular case, because those were guns from the 30's and marked 2 1/2".
Pooch, if you can post a shot of the barrel flats, or at least tell us everything that's marked on them, we may be able to help a bit more re dating the gun. Sounds as if the chambers might have been lengthened in the States. Assuming it's original 2 1/2", it would have had to undergo reproof if lengthened in England. One trick there is that the Brits use .800 chamber reamers. Ours are .797. So if you determine it's been lengthened and a .800 plug gauge stops short, you know it was done in the States.
