When annuity otherwise provided in this Article for the widow of an employee whose death results from injury incurred in the performance of an act of duty is less than 60% of his salary in effect at the time of the injury, “Compensation Annuity” equal to the difference between such annuity and 60% of such salary, shall be payable to her until the date when the employee, if alive, would have attained age 65. The county shall contribute to the fund each year the amount required for all compensation annuities payable during any such year.
     Thereafter, the widow shall be entitled to “Supplemental Annuity” equal to the differences between the annuity otherwise provided her in this Article and the annuity to which she would be entitled if the employee had lived and continued in service at the salary in effect at the date of the injury until he attained age 65, and based upon her age as it would be on the date he would have attained 65. Supplemental Annuity shall be provided from county contributions after the date of the employee’s death, of such equal amounts annually which when improved by interest at the effective rate, will be sufficient, at the time payment of Compensation Annuity to the widow ceases to provide Supplemental Annuity, as stated, for the widow throughout her life thereafter.