(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 and city contributions had been made in accordance with this Article during his entire period of service until he attained age 57, is entitled to an age and service annuity and prior service annuity from the date he withdraws from service, fixed as of his age on the date when he has to his credit such sums; such annuities shall be the amounts provided from the entire sum accumulated to his credit for age and service annuity and prior service annuity purposes on such date of fixing.
     (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 an age and service annuity and prior service annuity fixed as of his age on the date of withdrawal of the amount provided from the entire sum accumulated to his credit for age and service annuity and prior service annuity on such date of withdrawal.