Where a proved Barrel of One Sort which has by either of the Two Companies been proved and marked as proved definitively only under the recited Act or this Act, or which has not been proved and is not liable to be proved 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 for the Purposes of this Act be deemed an unproved Barrel.