It shall be unlawful (a) in or in connection with any transaction or shipment in commerce made after August 1, 1923, or (b) in any publication of a price or quotation determined in or in connection with any transaction or shipment in commerce after August 1, 1923, or (c) in any classification for the purposes of or in connection with a transaction or shipment in commerce after August 1, 1923, for any person to indicate for any cotton a grade or other class which is of or within the official cotton standards of the United States then in effect under this chapter by a name, description, or designation, or any system of names, description, or designation not used in said standards: Provided, That nothing herein shall prevent a transaction otherwise lawful by actual sample or on the basis of a private type which is used in good faith and not in evasion of or substitution for said standards.