Jim, I may have been the person you are quoting. About 2 years ago I was requested by the owner of a family gun to have it nitro re-proofed as he wanted to export it to his son in the USA and it is illegal to export a shotgun from the UK if out of proof.
The barrel walls had patches that measured 0.014" and 0.015" respectively about 18" from the breech.
It sailed through 2 1/2" nitro proof and as you may be aware there is no pressure difference between standard 2 1/2" and standard 2 3/4" proof, it is just a matter of chamber length.
