Whenever any standard for the grade or other classification of any farm product becomes effective under this article no person shall thereafter pack for sale, offer to sell or sell within this State any such farm product to which such standard is applicable unless it conforms to the standard, subject to such reasonable variations therefrom as may be allowed in the rules and regulations made under this article. But any farm product may be packed for sale, offered for sale or sold without conforming to the standard for grade or other classification applicable thereto if it is especially described as not graded or plainly marked "Not graded."