Paragraph (5) of section ninety-seven of the Explosives Act, 1875 (which exempts from that Act the conveyance of explosives under the control of a government department or otherwise held for the service of the Crown), shall be deemed to extend to any explosive the conveyance of which is certified by a government department to be in connection with the execution of a contract with that department and which is conveyed in accordance with regulations made under the said paragraph.