Government Code § 31510.4
ARTICLE 2.1. Additional Plan for Counties of the First Class
It is intended that the maintenance of plan F not affect the rate of either member or employer contributions to the retirement system. The board may set a rate of regular interest credited to contributions to the prior plan made with respect to members participating in plan F that is different than the rate of regular interest credited to contributions made with respect to members not participating in plan F, if necessary to effectuate that intent.