In every county in which a retirement system was established prior to January 1, 1953, if a member enters or has heretofore entered the retirement system with credit for prior service, and has heretofore retired or shall hereafter be retired after attaining the compulsory retirement age, or at age 65 with 20 years service, and his retirement allowance is less than one thousand two hundred dollars ($1,200) a year, exclusive of any annuity based on additional contributions, an additional amount purchased by contributions of the county or district shall be added to his combined annuity and pensions which will cause his total retirement allowance to amount to one thousand two

hundred dollars ($1,200) a year, exclusive of any annuity based on additional contributions.