Assessment non-filers: officer may assess to best judgment; valid return within period withdraws assessment but interest and late fee continue. A proper officer may assess a registered person who fails to furnish the return to the best of his judgement on the basis of all relevant material and issue an assessment order within five years from the date specified for the annual return; if a valid return is furnished within thirty days of service of the assessment order it is deemed withdrawn, but interest and late fee liabilities continue.