Assessment of non-filers: officer may assess to best judgement; later filing can withdraw assessment but interest and fees remain. The proper officer may assess the tax liability of a registered person who fails to furnish returns after notice to the best of his judgement, using all relevant material and issuing an assessment order within the statutory assessment period measured from the annual return due date; if a valid return is filed within the short period after service of the assessment order, the assessment is deemed withdrawn but interest and late fee liabilities continue to apply.