Carcasses and parts therefrom that are prepared on a custom basis under § 303.1(a)(2) of this subchapter shall be marked at the time of preparation with the term “Not for Sale” in letters at least three-eighths inch in height, except that such products need not be so marked if in immediate containers properly labeled in accordance with the regulations in § 317.16 of this subchapter. Ink used for marking such products must comply with the requirements of § 316.5.