The first three hundred dollars ($300) of the monthly allowances payable to retired members, disabled members, and beneficiaries are increased as of July 1, 1976, as follows if the member had 20 or more years of credited service:
(a) For those with effective dates prior to July 1, 1972, 9 percent.
(b) For those with effective dates from July 1, 1972, to June 30, 1973, 6 percent.
(c) For those with effective dates from July 1, 1973, to June 30, 1974, 3 percent.