Where a person is entitled to additional retiring allowance under subsection 18(9) in respect of an office in respect of which a salary or an allowance by way of salary was not or is not being paid at any relevant time, whether for the reason that there was not or is not any such office at that time or for any other reason, then, for the purposes of this Act, salary or allowance by way of salary, as the case may be, shall be deemed to have been or to be paid in respect of that office at that time at such annual rate as the Minister determines by instrument in writing.