50. Where the amount paid by an operator as duties payable for a fiscal year, before the expiry of the time allowed for filing his return, is less than the amount of the duties payable for that year, the person liable to pay the duties shall pay interest at the rate fixed by regulation on the difference between those two amounts, from the expiration of the delay for filing the return to the day of payment; if no amount has been paid by the operator, such interest is exigible on the total amount of duties payable for the same period.