(1) The rate of duty and tariff valuation, if any, applicable to any export goods, shall be the rate and valuation, in force,-
(a) in the case of goods entered for export under section 50, on the date on which the proper officer makes an order permitting clearance and loading of the goods for exportation under section 51;
(b) in the case of any other goods, on the date of payment of duty".]