31 Pa. Stat. § 841
Every person who shall manufacture for sale, or who shall offer or expose for sale, or who shall export to a foreign country, by the tub, firkin, box or package, or any greater quantity, any article or substance in semblance of butter or cheese, not the legitimate product of the dairy, and not made exclusively of milk or cream, but into which any oil, lard or fat not produced from milk or cream enters as a component part, or into which melted butter or any oil thereof has been introduced to take the place of cream, shall distinctly and durably stamp, brand or mark upon the side of every cheese, and also upon the top and side of every such tub, firkin, box or package of such article or substance, the words "oleomargarine butter," or if containing cheese the words "imitation cheese" only, where it can be plainly seen in Roman letters, which shall be burned in or painted thereon with permanent black paint in a straight line, and shall not be less than one-half inch in length, and if for export shall also invoice the same and clear the same through the custom house as "oleomargarine butter," or if cheese, as "imitation cheese," and in case of retail sales of such articles or substances in parcels, the seller shall, in all cases, sell or offer or expose the same for sale from a tub, firkin, box or package stamped, branded or marked, as herein stated, and shall also deliver therewith to the purchaser printed label bearing the plainly printed words "oleomargarine butter," or if cheese, the words "imitation cheese" only, in Roman letters not less than one-half inch in length, which shall be printed in a straight line, and every sale of such article or substance, or export of the same, by the tub, firkin, box or package, or in any greater quantity, not so stamped, branded or marked, and, if exported, not invoiced and cleared through the custom house as "oleomargarine butter," or if cheese as "imitation cheese," and every sale of such article or substance at retail in parcels that shall not be sold from a tub, firkin, box or package so stamped, branded or marked, or without delivery of a label therewith as above stated, is declared to be unlawful and void, and no action upon any contract shall be maintained in any of the courts of this state to recover upon any contract for the sale of any such article, or semblance, not so stamped, branded, marked, labeled or sold.
31 P.S. § 841