(1) In computing the amount of tax payable by an accountable person, the following amounts may, subject to subsections (3) and (4), in addition to the deductions authorised by section 12, be deducted on account of stock-in-trade which has been supplied to, and has not been supplied by, him before the specified day and which is held by him at the commencement of that day, or incorporated in other stock-in-trade held by him at such commencement, that is to say:
(a) in case the accountable person was, immediately before the specified day, not registered for turnover tax under the provisions of section 49 of the Finance Act, 1963, nor required under the provisions of that section to furnish the particulars specified for registration, and was not registered for wholesale tax under the provisions of section 4 of the Finance (No. 2) Act, 1966, nor required under the provisions of that section to furnish the particulars specified for registration, an amount equal to the sum of the amounts