Assessment of unregistered persons: officers may estimate tax liability and issue orders after hearing within a five-year period. Where a taxable person fails to obtain registration despite being liable, or whose registration has been cancelled but who remained liable to pay tax, the proper officer may assess that person's tax liability to the best of his judgment for the relevant tax periods and issue an assessment order, provided the assessment is issued within five years from the date specified for furnishing the annual return for the relevant financial year and the person is given an opportunity of being heard.