Notwithstanding the provisions of Kan. Stat. Ann. §§ 74-4913 to the contrary, a county employee who first became a member of the Kansas public employees retirement system on October 1, 1964, having been first employed on October 1, 1963, by a participating county employer which affiliated with the retirement system on January 1, 1962, having over fifteen (15) years prior service with an adjoining county which became a participating employer on January 1, 1968, shall receive prior service credit for employment with any participating employer for any employment performed before January 1, 1962, whether such employment was continuous or not. A prior service annual salary of four thousand two hundred dollars ($4,200) shall be used in computing the prior service benefit.