The average monthly salary during any period of service as a member of a county retirement system shall be considered compensation earnable by a member of this system for purposes of computing final compensation for the member provided:
(a)
(1)Entry into employment in which he or she became a member in one system occurred on or after October 1, 1957, and within 90 days of discontinuance of employment as a member of the other system.
(2)
This subdivision shall not deny the benefit of this section to any person retiring after October 1, 1963, who entered membership prior to October 1, 1957, if he or she entered the employment in which he or she became a member within 90 days of termination of employment in which he or she was a member of the other system, and he or she became a member within seven months of entry into employment, or, if an employee of a district as defined in Section 31468, became a member at the time the district was included in a county retirement system .
(b)
He or she retires concurrently under both systems and is credited with the period of service under the county system at the time of retirement.