Notwithstanding the provisions of Kan. Stat. Ann. §§ 74-4913 to the contrary, an employee who was employed by a participating employer on April 1, 1964, who became a member of the Kansas public employees retirement system on April 1, 1965, who was previously employed by a participating employer whose entry date was January 1, 1967, and by whom he was employed from October 2, 1956 to April 30, 1964, shall upon retirement receive prior service credit for such service. The prior service benefit shall be computed by using a prior service annual salary of four thousand one hundred twenty-eight dollars and twenty-three cents ($4,128.23).