All persons who were teachers or State, county or municipal employees on April 26, 1945, or who became such after this date but on or before December 31, 1966, except those specifically excluded under § 9-1-450 and the persons permitted to exercise the option under §§ 9-1-510 to 9-1-560, became members as of July 1, 1945, or as of the date of last employment, if later, unless on or before December 31, 1948, they filed with the Board on a form prescribed by the Board a notice of their election not to be covered in the membership of the System and a duly executed waiver of all present and prospective benefits which would otherwise inure to them on account of their participation in the System.