Provided always, that whenever any Barrel proved and marked as proved provisionally by either of the Two Companies under the recited Act or this Act, and not being a Breech Loader, is brought to the Proof House or a Branch Proof House of the respective Company by which the Barrel was originally so marked, accompanied by a Statement in Writing made or purporting to be made by or on behalf of the Owner or Owners of the Barrel that the same is intended to be converted into a Breech Loader, the Proof Master of the Gunmakers Company, or the duly appointed Officers in that Behalf of that Company, or, as the Case may be, the Proof Master or Assistant Proof Masters of the Guardians, may efface all existing Proof Marks from the Barrel, and, where it is not necessary that the Barrel be again submitted to provisional Proof, may and shall impress a new provisional Proof Mark at a proper and convenient Distance higher up on the Barrel, without subjecting the Barrel to any further provisional Proof.