It shall be unlawful to purchase or receive cream for buttermaking purposes except on the basis of the following grades: Sweet cream, first grade and second grade. Every person shall buy or receive cream on a grade basis, and each grade shall be kept and shipped so as to arrive within forty-eight (48) hours after the date of purchase at a dairy products manufacturing plant. The cream shall be shipped in a separate can plainly marked to indicate the grade therein and the date graded, and the person buying or receiving the cream shall maintain a price differential between grades on a recognized established differential. This differential shall be a minimum of two cents (2¢) per pound of butterfat between first grade cream and second grade cream.