Notwithstanding the provisions of Kan. Stat. Ann. §§ 74-4913 to the contrary, if any employee of a participating employer who first became a member of the Kansas public employees retirement system on February 1, 1965, who was first employed by a participating employer on September 16, 1947, through January 28, 1961, and was employed by another participating employer on January 1, 1961, through December 31, 1963, and was employed by another participating employer on February 1, 1964, through January 12, 1970, shall upon retirement be granted prior service credit for said service prior to January 1, 1964. The prior service benefit shall be computed by using a prior service annual salary of six thousand seven hundred fifty dollars ($6,750).