Interest on delayed duty applies for collusion or willful suppression, charged from the month after payment became due until paid. Insertion of section 28AB imposes interest liability where duty was not levied, short levied, or erroneously refunded due to collusion, willful mis-statement, or suppression of facts; interest is charged at a Board-fixed rate within prescribed bounds from the month after duty became payable or from the date of erroneous refund until payment, is prospective only and excludes duties payable before assent, and is adjusted to reflect reductions or increases ordered on appeal or by a court.