Notwithstanding the provisions of Kan. Stat. Ann. § 74-4913, any county treasurer who has elected to become a member of the Kansas public employees retirement system and who was elected to office at the general election next preceding the fifteenth day of March, 1961, but who had not yet taken office on said fifteenth day of March, 1961, with the employer who was his employer on entry date, then all such previous employment, with said entry date employer whether continuous or not, shall be credited. The prior service annual salary in such case shall be calculated by multiplying the monthly compensation paid immediately prior to entry date times twelve (12).