Present employees – Limitation to and amount of prior service annuities in certain cases. A present employee who has a credit on the effective date, for prior service annuity, of an amount sufficient to provide annuity as of his age on such date equal to that to which he would have had a right if employee contributions by salary deductions and city contributions had been made for age and service annuity during his entire period of service until his attainment of age 57, is entitled to a prior service annuity from the date he withdraws from service of such amount as can be provided by his credit for prior service annuity on the effective date. Any such present employee has no right to receive age and service annuity.