30.—(1) Every ballot paper shall bear an appropriate security marking (“the official mark”).
(2) The official mark shall be kept secret, and an interval of not less than seven years shall intervene before the same official mark may be used again at a poll for a Scottish parliamentary election held in that constituency.
(3) The CRO shall determine the official mark or marks to be used for ballot papers at an election and may use a different official mark for different purposes at the same election.