A present employee who was a contributor to a county pension fund in operation on the effective date who withdraws on or after such date having 20 or more years of service and for whom the amount of annuity provided by this Article is less than the amount stated in this section has a right to receive annuity as follows:
In addition to the combined age and service and prior service annuities to which a present employee is entitled, an employee with 24 or more years of service who has attained age 65 or more at the time he withdraws is entitled to receive a sum equal to the difference between the combined age and service annuity and prior service annuity, and 1/3 of his salary at the date of his withdrawal.
40 ILCS 5/9-132