Section 23(5) of the Income tax Act, as it stood at the material time, read as follows : "(5) Notwithstanding anything contained in the foregoing sub sections, when the assessee is a firm and the total income of the firm has been assessed under sub section (1), sub section (3), or sub section (4) as the case may be. (a) in the case of a registered firm, the sum payable by the firm itself shall not be determined but the total income of each partner of the firm, including therein his share of its income, profits and gains of the previous year, shall be assessed and the sum payable by him on the basis of such assessment shall be determined Provided Provided further Provided also (b) in the case of an unregistered firm, the Income tax Officer may instead of determining the sum payable by the firm itself proceed in the manner laid down in clause (a) applicable to a registered firm, if in his opinion, the aggregate amount of the tax including super tax, if any, payable by the partners under such procedure would be greater than the aggregate amount which would be payable by the firm and the partners individually if the firm were assessed as an unregistered firm.