13Of the persons elected members of the Council at the election of such members in 1968 one shall retire in each of the three next following years, being—
(a)that one of the successful candidates who at that election received the smallest number of votes and remains a member by virtue of being so elected, or
(b)if two or more such candidates received an equal number of votes, the candidate who has been registered on the register for the shortest period or, if two or more such candidates have been registered for a shorter period than any other but for the same period as each other, one of them chosen by lot."