If the commission finds that the sum of (a) any retirement allowance payable to a member under section 7-436 and (b) any concurrent benefit payable to such member under the Old Age and Survivors Insurance System on account of service in a participating municipality is less than the allowance that would have been payable if the member were not participating in such system, his allowance shall be increased accordingly during the period when the lower sum would otherwise be payable. When any person retires under the terms of section 7-436 prior to the age at which he would be eligible to receive an old age insurance benefit under the system, he shall be entitled to receive an additional temporary retirement allowance payable to the age at which he would be so eligible, or payable until he qualifies for a social security disability allowance, if earlier. The total retirement allowance payable during the temporary period shall be computed as if such person was not covered by the Old Age and Survivors Insurance System. The first sentence of this section shall not apply to any employee who was hired by a municipality after the date when the municipality included its employees in such system, or to any employee who transfers from a nonparticipating department to a participating department of a municipality after said date, or to any employee of a municipality which had included its employees in such system not later than the effective date of participation as defined in section 7-427.