U.S. Customs has provided further guidance on documents they may require when a review for duty free claims under 9801.0010 is made.  A significant change occurred as a result of the Trade Facilitation and Trade Enforcement Act of 2015, which allows for US origin products returned within any time frame after having been exported, and allowing for products of other countries returned within three years have having been exported, and not advanced in value or imported in condition are now eligible for duty free treatment.  U.S. Custom Guidance: https://apps.cbp.gov/csms/viewmssg.asp?Recid=22450&page=&srch_argv=&srchtype=&btype=&sortby=&sby