Notwithstanding the provisions of Kan. Stat. Ann. §§ 74-4913 to the contrary, an employee of a participating employer who became a member of the Kansas public employees retirement system on January 1, 1968, and who was previously employed from December 7, 1956 until December 16, 1966, by another participating employer with a January 1, 1970 entry date, shall upon retirement be granted prior service credit for all service with said employer prior to January 1, 1970. The prior service benefit shall be computed by using a prior service annual salary of two thousand eight hundred and twenty dollars ($2,820).