Notwithstanding the foregoing provisions of this section, where any person has elected to accept a variable retiring allowance and there has been no reduction of the rate for the time being payable in respect of that allowance, that person may, if the Board so permits, further elect, at any time not later than the 31st day of December 1979, to have an actuarial adjustment of that allowance so that the reduction in the rate of payment will become effective on such date as the Board may determine, being a date earlier than the date on which the person will attain the age of 65 years.”