Provisional assessment permits provisional duty payment with bond; final assessment, interest and refund rules then apply. The rule permits provisional payment of duty when value or rate cannot be determined, upon written request and payment at a rate or value specified by the officer, conditional on executing a prescribed bond with surety or security to cover any difference on final assessment. Final assessment must be completed within the prescribed period subject to extensions for sufficient cause, interest is payable on unpaid provisional amounts after the due date, and refunds on final assessment are credited to the Fund unless attributable to duty not passed on, in which case refund may be paid to the applicant.