Where a superintendent of schools in a district of the first class, has been employed in the public schools of the Commonwealth for fifteen or more years, and is retired from service under the operation of the State School Employes' Retirement System, the board of public education may, when the superintendent of schools has previously served as a teacher, principal, or superintendent of schools, in public schools in the United States of America outside the Commonwealth, provide for the payment to the superintendent of schools annually, during his retirement, of a sum equal to the difference between his annual retirement allowance, based on the service in the public schools of the Commonwealth and according to his optional election at the time of retirement, and the annual allowance to which he would have been entitled had he been given credit for his period of service as teacher, principal, or superintendent of schools, both outside and within the Commonwealth. A superintendent of schools when thus retired may be designated by the board as superintendent emeritus.