Assessment of unregistered persons allows best-judgment tax liability determination within prescribed period, subject to right to be heard. Where a person liable to register fails to obtain registration or has had registration cancelled but remained liable to pay tax, the proper officer may assess tax to the best of his judgment for the relevant periods and issue an assessment order within the prescribed limitation period measured from the due date for filing the annual return for the financial year to which the unpaid tax relates, provided the person is given an opportunity of being heard.