In counties adopting Section 31676.11, 31676.13, or 31676.14, or adopting one of such sections and then subsequently adopting another of such sections, the normal rate of contribution, except for members covered by Article 6.8 (commencing with Section 31639) of this chapter shall be such as will provide an average annuity at age 55 equal to one two-hundred-fortieth of the final compensation of members not covered by Article 6.8, according to the tables adopted by the board of supervisors, for each year of service rendered after entering the system.

This section may be made applicable in such counties on the first day of the month

after the board of supervisors of such county adopts, by majority vote, a resolution providing that this section shall become applicable in such county.