Assessment of unregistered persons permits best-judgment tax assessment where registration absent, subject to hearing and time limits. Where a person liable to be registered fails to obtain registration or has registration cancelled under section 29(2) but remained liable to pay tax, the proper officer may assess tax to the best of his judgement for the relevant periods, notwithstanding sections 73 and 74, and issue an assessment order within the time limit measured from the date under section 44 for filing the annual return; the person must be given an opportunity of being heard.