Chewing tobacco packing machine duty must be paid monthly; interest, pro rata adjustments, differentials and refunds prescribed. Rule 13 requires monthly duty to be paid by the 6th and a return filed, with FORM CE PMT-01 by the 10th; interest applies to unpaid amounts from the day after the due date. Differential duty for added or faster machines is payable within five days. Pro rata recalculation applies on discontinuation, commencement of new retail sale prices, or duty rate revision, with differentials due by the 6th of the following month and refunds by the 20th. New manufacturers pay pro rata within five days. Undeclared or contravening manufacture attracts duty at the highest retail sale price for all machines during the period.