15.Four forms of certificates shall be issued, designated as follows:
(1) Certificate “A” issued for the approval of the construction of pressure vessels after verification of all the plans and specifications and final inspection of the said vessels at the place where they are manufactured;
(2) Certificate “B” issued for the approval of the installation of pressure vessels prior to their being utilized at the place where they are to be operated;
(3) Certificate “C” issued on the annual inspection of pressure vessels;
(4) Certificate “D” issued for any used vessel before it is again dealt with in trade.