Section 20. Whoever himself or by his servant or agent sells, exchanges or delivers, or has in his custody or possession with intent so to do, milk not conforming to the Massachusetts standard shall be punished for the first offence by a fine of not more than fifty dollars, for the second offence by a fine of not less than one hundred nor more than two hundred dollars, and for a subsequent offence by a fine of not less than one hundred nor more than two hundred dollars or by imprisonment for not more than three months; provided, that it shall not constitute a violation of this section for a producer of milk operating a dairy farm registered under section sixteen C to sell, or deliver in exchange or otherwise, to a dealer registered under section sixteen F natural cow’s milk produced on such dairy farm and not conforming to the state standard, or for such a producer to have in his custody or possession any such milk with intent to sell, or deliver in exchange or otherwise, such milk to such a dealer, or for such a dealer to have in his custody or possession such milk, obtained as aforesaid from such a producer, with intent to blend such milk with other natural cow’s milk for the purpose of making the blend conform to said standard.