The widow of a present employee or future entrant who dies in service is entitled to receive annuity, from the date of his death, of the amount provided from the sums to his credit at his death for age and service annuity, widow’s annuity, and, if a present employee, prior service annuity and widow’s prior service annuity; but no part of such sums so credited which represent contributions by the city shall be used to provide annuity for the widow in excess of that to which she would have had a right if the employee had lived and remained in service at final salary until age 63 or until such date as he would have retired by operation of law, whichever is later, and the amount of annuity for his wife were then fixed. The annuity shall be computed as of the widow’s age on the date of the employee’s death.