19 C.F.R. § 207.103
(a) Contents of charging letter. Each charged party shall be served by the Commission with a copy of a charging letter and any accompanying motion for interim measures, as provided for in 19 CFR 207.106. The charging letter shall include:
(b) Service of charging letter.
(1) The charging letter shall be served in a double envelope. The inner envelope shall indicate that it is to be opened only by the addressee. Service of a charging letter shall be made by one of the following methods:
(d) Amendment of charging letter.