The supplemental pension paid under section 15a of chapter 1 or section 57 of chapter 2 of former Act No. 136 of the Public Acts of 1945 shall continue to be paid to a retirant or retirement allowance beneficiary. The supplemental benefit, when added to the retirement allowance of the retirant or retirement allowance beneficiary, shall provide before the election of an option or early retirement a minimum retirement allowance of $160.00 for each year of credited service on the date of retirement or death adjusted to the nearest number of even years.