If there is exposed from, maintained in or permitted to remain on any vehicle or premises any placard, sign or advertisement purporting or designed to announce that firearms, rifles, shotguns or machine guns are kept in or upon such vehicle or premises or that an occupant of any vehicle or premises is a gunsmith, it shall be prima facie evidence that firearms, rifles, shotguns or machine guns are kept in or upon such vehicle or premises for sale or that the occupant is engaged in business as a gunsmith.
Massachusetts Legal Code