A policeman who was not in the police service of the city on the day prior to the effective date, and who was in such service prior to that date, and who re-enters the service after that date and before age 57, and who withdraws after he has completed at least 20 years of service (at least 5 years of which shall be subsequent to his re-entry), and who has contributed to this fund or to its predecessor fund, or both, for a period of at least 20 years, and for whom the amount of annuity provided in accordance with foregoing provisions of this Article is less than the amount stated hereinafter in this section, shall have a right to receive annuity as follows:
     (1) If he is age 50 or more when he withdraws, his annuity, payable from the date of withdrawal, shall be 50% of the compensation for the rank in the police department which he held by civil service appointment on the day one year prior to the date of withdrawal; or
     (2) If he is less than age 50 when he withdraws, his annuity, payable from the date he attains age 50, shall be 50% of his salary at the time of withdrawal but not in excess of $900 a year.