(a) Articles acquired abroad. Articles subject to internal revenue tax and other merchandise acquired abroad constituting supplies arriving on international trains crossing and recrossing the boundary line, for which the train crew elects not to file an inventory as provided for in paragraph (b) of this section, shall be subject to duty and tax unless locked or sealed in a separate compartment or locker upon arrival, and the lock or seal remains unbroken until the train departs from the United States at the final port of exit.
(b) Inventory procedure. Supplies acquired abroad for which internal revenue stamps are not required may be used in the United States under the following procedure:
(1) Port of arrival. An inventory executed in duplicate consisting of an itemized list showing the kind and quantity of each class of supplies on hand in the car with space for a parallel column in which to show at the port of exit the quantity used, shall be certified by the person in charge of the car and furnished to the Customs officer upon arrival. The Customs officer shall certify the correctness of both copies of the inventory, return the original to the person in charge of the car and retain the duplicate, or forward it to the port of exit if this differs from the port of arrival.
(2) Port of exit. Upon arrival at the port of exit, the inventory returned at the port of arrival to the person in charge of the car shall be submitted to the Customs officer after completion by showing the quantity of each item used in the United States, and being certified by the person in charge of the car. Entries must be filed and applicable duties and taxes paid at the port of exit on the quantity of supplies consumed in the United States.
(c) Supplies purchased in the United States. Supplies purchased in the United States shall be passed free of duty without inventory or entry.