Notwithstanding any other provision of this part, credit for prior service shall be granted only to each person who was employed by a contracting agency on the effective date of the agency’s contract or amendment to the contract under which that prior service is granted.

This section shall not apply to a contracting agency nor to the employees of a contracting agency, unless the agency elects to be subject to this section by express provision in the contract making the contracting agency subject to this section.