In Section 31641.2 compensation first earnable shall be that earnable by the member at the time of the commencement of his membership in the retirement system, except that if a member who, prior to January 1, 1952, was a member of any retirement system in the same county which was superseded by a system established under this act, if such member so requests, the compensation first earnable by him at the later of the following times shall be used:
(a) The commencement of his membership in such superseded retirement system.
(b) The date on which this retirement system first became operative.