Assessment of non-filers: officer may assess tax to best judgement; subsequent valid return can withdraw order though interest remains. Proper officers may assess the tax liability of registered persons who fail to furnish returns to the best of his judgement using available material and must issue the assessment within five years from the date specified for filing the relevant annual return. If a valid return is furnished within thirty days of service of the assessment order, the assessment is deemed withdrawn, while liability for interest and late fee remains.