Every employee of a child development program who before his employment in such program was employed by the agency maintaining such program in a position entitling him to membership in, and who was a member of, the retirement system maintained by such district, and if such employee’s contributions to such retirement system were returned to him when he was employed in the program, such employee shall have the right to elect, by written document filed with the Board of Administration, Public Employees’ Retirement System, at any time within 90 days after the date upon which the notice of the right to make such election is mailed by such system, either to the member’s latest address on file in the office of such system, or to the office of the governing board of such agency or agencies, and prior to the date of retirement, to contribute to such system, subject to minimum payments fixed by the Board of Administration, and in one or more sums, or in not to exceed 60 monthly payments, an amount which, when added to his accumulated contributions, including interest, transferred as required in paragraph (1) of subdivision (b) of Section 24810, will make a total amount equal to the accumulated contributions, including interest, which would have been credited to him in such plan, if he had never had his contributions returned to him. Such employee shall pay to the Public Employees’ Retirement System interest on the unpaid balance of the amount payable to such system, beginning with the date of transfer, at the rate of interest currently used from time to time under the system. If such employee elects to make, and makes, such contributions and pays such interest, but not otherwise, he shall receive credit under such employees’ system, as state service, for all prior service rendered while he was not a member of such plan.