Notwithstanding anything contained in this Act, the articles of a company may provide for the appointment of not less than two-thirds of the total number of the directors of a company in accordance with the principle of proportional representation, whether by the single transferable vote or by a system of cumulative voting or otherwise and such appointments may be made once in every three years and casual vacancies of such directors shall be filled as provided in sub-section (4) of section 161.