Assessment of non-filers permits best-judgement tax assessment; a timely valid return withdraws the order but interest remains. Where a registered person fails to furnish a required return after notice, the proper officer may assess tax to the best of his judgement using all relevant material and must issue the assessment within the limitation period measured from the date for filing the annual return; if a valid return is furnished within the short period after service of the assessment order, the order is deemed withdrawn but interest and late fee liabilities continue.