Containers of 1 gallon (3.785 liters) or less of distilled spirits, upon which all Federal internal revenue taxes have been paid or deferred in Puerto Rico under provisions of this part, shall have closures or other devices affixed in accordance with the provisions of this part, prior to shipment to the United States.
27 C.F.R. § 26.135
Sec. 454, Pub. L. 98-369, 98 Stat. 494 ( 26 U.S.C. 5301 )