Notwithstanding the provisions of Kan. Stat. Ann. §§ 74-4913 to the contrary, an employee who became a member of the Kansas public employees retirement system on April 1, 1965, who was first employed by the same participating employer from March 1, 1957, to September 30, 1961, shall, upon the effective date of this act, be entitled to receive prior service credit for said employment prior to January 1, 1962. The prior service benefit shall be computed by using a prior service annual salary of three thousand five hundred sixteen dollars ($3,516).