(a) As an additional supplement to other retirement allowances provided, each annuitant whose annuity was approved by the retirement board prior to the first day of January, one thousand nine hundred seventy-one, and who is receiving a monthly pension of three hundred dollars or less, shall receive a monthly amount equal to one dollar multiplied by his or her total service credit.
(b) As an additional supplement to other retirement allowances provided, each annuitant whose annuity was approved on or after the first day of July, one thousand nine hundred eighty-two, and before the first day of July, one thousand nine hundred eighty-four, shall receive a monthly amount equal to two dollars multiplied by his or her total service credit.