Holt’s will (or did) submit seller’s guns for proof, as I remember when they were holding their auctions in Hammersmith and looking for a gun in my list of things to see being told “it’s still at the Proof House” but that was generally rifles and relatively straightforward for clean barrels that being ex-service or personal imports lacked a recognised civilian proof.
If they felt that a vendor had a set of barrels that were sufficiently sound to justify the expense, delay and risk to the rest of the gun I am sure they could arrange for a gunsmith to do the work IF the customer wanted.
Often the economics and the end result in added value will still rule against that course. What might make sense for a Boss or Purdey might not for a less valuable make, even if of equal quality.
