Where a proved Barrel of One Sort which has been proved and marked as proved provisionally by either of the Two Companies under the recited Act or this Act, and whether or not it has been proved and marked as proved definitively by the respective Company under the recited Act or this Act, is converted into a Barrel of another Sort (as, for instance, Flint into Percussion, or Muzzle Loader into Breech Loader, or vice versa), the Barrel from the Time when the Conversion thereof is begun shall, if when the Conversion of it is completed the Mark of provisional Proof be upon it, be deemed for the Purposes of this Act a Barrel proved provisionally by the Company whose provisional Proof Mark it bears.