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, 1967, and who was previously employed by said employer on July 1, 1947 to December 29, 1950, shall upon retirement be granted prior service credit for all service with said participating employer prior to January 1, 1962. The prior service benefit shall be computed by using a prior service annual salary of two thousand six hundred forty dollars ($2,640.00).