The official mark
30.—(1) Every ballot paper shall be marked with an official mark, which shall perforate the ballot paper.
(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 official mark used for ballot papers issued for the purpose of voting by post shall not be used at the same election for ballot papers issued for the purpose of voting in person.