The widow of an employee with 10 or more years of service who withdraws before age 55 and who dies while out of service before age 55 shall be entitled to an annuity computed on a single life annuity basis at the date of death from the following credits:
(1) If service is 20 or more years, the total credits for age and service annuity and widow's annuity, and, in addition, if he was a present employee, the total credits for prior service annuity and widow's prior service annuity; or(2) If service is 10 or more but less than 20 years, the total credits for age and service annuity, and widow's annuity from employee contributions, and in addition, if he was a present employee the total credits for prior service annuity and 1/10 of the total credits for age and service annuity and widow's annuity from city contributions for each year of service after the first 10 years, including, for the widow of a present employee, 1/10 of the credits for prior service and widow's prior service annuity from city contributions for each year of service after the first 10 years. No city contributions shall be used for a widow's annuity in excess of that which she would receive if the employee had lived until he attained age 55 and had not reentered the service and an annuity were fixed for her on a reversionary annuity basis, as of her age when her husband would have attained age 55.