As used in this section, “county historical society” means a private, non-profit organization exempt from federal income taxation pursuant to section 501 (a) and (c)(3) of the Internal Revenue Code, 26 U.S.C.A. 501(a) and (c)(3), as amended, that collects, preserves, and interprets the historical physical and intellectual resources of a county.

An administrative employee of a county historical society who is a contributor on the effective date of this section may elect to remain a contributing member of the public employees retirement system by giving notice to the system not later than ninety days after the effective date of this section. The election once made is irrevocable.