When any person occupying the position of superintendent of buildings in a district of the first class, who is under the super-annuation retirement age fixed by the statutes governing the State School Employes' Retirement System, but who has been employed in the public schools of the Commonwealth continuously for thirty years or more, is retired under the operation of that system because of disability, the board of public education may provide for the payment to such superintendent of buildings annually during his retirement, of a sum equal to the difference between the annual retirement allowance actually paid to him under the State School Employes' Retirement System according to his optional election at the time of retirement, and the annual allowance to which he would have been entitled under said system had he served, at the salary paid to him at the date of his retirement, until superannuation age and then retired under an election to receive his benefits by way of an undiminished superannuation retirement allowance payable to him throughout life.