19 CFR 10.138 : Customs Duties (§ 0.1 To Parts 400-599) — (§ 0.1 To Parts 135-140) — U.S. Customs And Border Protection, Department Of Homeland Security; Department Of The Treasury (§ 0.1 To Parts 135-140) — Articles Conditionally Free, Subject To A Reduced Rate, Etc. (§ 10.1 To § 10.3034) — General Provisions (§ 10.1 To § 10.183) — Rate Of Duty Dependent Upon Actual Use (§ 10.131 To § 10.139) — Proof of use.

Within 3 years from the date of entry or withdrawal from warehouse for consumption, the importer shall submit in duplicate in support of his claim for free entry or for a reduced rate of duty a certificate executed by (1) the superintendent or manager of the manufacturing plant, or (2) the individual end-user or other person having knowledge of the actual use of the imported article. The certificate shall include a description of the processing in sufficient detail to show that the use contemplated by the law has actually taken place. A blanket certificate covering all purchases of a given type of merchandise from a particular importer during a given period, or all such purchases with specified exceptions, may be accepted for this purpose, provided the importer shall furnish a statement showing in detail, in such manner as to be readily identified with each entry, the merchandise which he sold to such manufacturer or end-user during such period.