(a) Notwithstanding Section 31491, any retired member receiving a retirement pension may present evidence in the form required by the board of the retired member’s actual primary insurance amount. For the purposes of this section, the actual primary insurance amount shall be the amount being paid under the federal system. Following receipt of that evidence, the board shall adjust the retired member’s pension to equal the amount of the pension to which he or she would have been entitled on that date had the estimated primary insurance
amount equaled the actual insurance amount.
(b) The adjustment calculated in subdivision (a) shall be applied to the retired member’s pension beginning in the month upon which the retired member presents evidence required by the board.
(c) This section shall not be operative in any county until the board of supervisors, by resolution adopted by majority vote, makes this section applicable.