Every retirement allowance payable for time commencing on the effective date of this section to, or on account of any member of this system or of a superseded system, who has been retired for service, is hereby increased as follows:

In no event shall any allowance be increased by an amount greater than fifty dollars ($50) a month nor less than ten dollars ($10) a month.

This section shall not be operative in any county until such time as the board of supervisors shall, by ordinance adopted by majority vote, make the provisions of this section applicable in such county.