On close inspection with a magnifier I can see that the chamber has been sleeved. There is no obvious seam but I can see a slight difference in the direction of the tooling marks between the outside of the barrel and a concentric ring on the inner side. The ejectors also have been reamed out and metal added so they are concentric with the rest of the chamber and there is a slight seam there.. It looks like a very professional job but it worries me that the gun is not in proof.
Your concern that the gun has not been re-proofed is prudent. If the gun was chamber sleeved by someone like Kirk Merrington, then one can assume with some reason that the chamber sleeve boring away of the original chamber was carried out in a workmanlike manner, and maybe it will stand up to pressures you submit the gun while firing it. If it was not Kirk or another of his caliber, then your worry continues as you cannot "see" how much of the barrel bore might have been cut away beyond the distance that it is safe to do.
What does the original seller to you say about this conundrum?
We do not have proof houses in the USA, and accordingly shotgun and rifle manufactures here use their own proof loads in their factory. I wish we had a USA proof house, but I suspect the lawyers do not.
With the last double rifle I built which had a service pressure of 38,000 psi I loaded heavy loads of 54,000 psi and shot two of these loads in each barrel (while I was behind a protective barrier). Even that does not make the rifle I built absolutely incapable of blowing the barrel, but it gave peace of mind somewhat.