All existing officers and employees of the county, now members of the retirement system, and who became employed by the county during that interim period when the law did not allow membership into the retirement system until the first day of the calendar month following the expiration of 180 days after his entrance into service, shall be allowed to make contributions and receive credit for that period of time as prior county service. The contribution rate shall be as prescribed in Section 31641.5 and election to receive credit for such service shall be available until time of filing of notice of retirement from county service.