(a) A present employee who attains age 57 or more while in service, having credit from sums accumulated for age and service annuity and prior service annuity sufficient to provide annuity as of his age at such time equal to that to which he would have had a right if employee contributions by salary deductions and city contributions had been made in accordance with this Article applicable to age and service annuity during his entire period of service until he attained age 57, is entitled to such age and service annuity and prior service annuity when he withdraws.
     (b) A present employee who attains age 57 or more while in service and who has not to his credit for age and service annuity and prior service annuity the amount described in paragraph (a) above is entitled on the date of his withdrawal to such age and service annuity and prior service annuity as can be provided by the amount to his credit for such annuities.