Widow’s annuity-All employees-Death in service before age 63. The widow of an employee who dies in service before age 63 is entitled to receive annuity, from the date of his death, of the amount provided on a single life annuity basis from the total sum accumulated to his credit at his death for age and service annuity, widow’s annuity, and if a present employee, prior service and widow’s prior service annuity; but no part of such credits which represent the city contributions shall be used to provide annuity for the widow in excess of the maximum widow’s annuity provided in this Article. The annuity shall be computed as of the date of the employee’s death.