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for time commencing on the effective date of this section to or on account of any member of this system, or of a superseded system, who has been retired for disability, is increased as follows: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, providing further that an actuarial survey of the system has been made by the adopting county’s system prior to the passage of said ordinance.