Carcasses and parts therefrom that are prepared on a custom basis under Sec. 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 Sec. 317.16 of this subchapter. Ink used for marking such products must comply with the requirements of Sec. 316.5. [35 FR 15577, Oct. 3, 1970, as amended at 38 FR 29214, Oct. 23, 1973]