Notwithstanding the provisions of Kan. Stat. Ann. § 74-4913, any clerk of the district court of any county who first became a member of the Kansas public employees retirement system on January 1, 1968, by filing the appropriate election to become a member of said system as required by Kan. Stat. Ann. § 74-4911(3) shall receive prior service credit for previous employment with any participating employer, whether or not continuous: Provided, That said clerk of the district court had been employed by the entry date employer for not less than fifteen (15) years.