Section 30. No person, firm, association or corporation shall post, erect, display or maintain on any public way or on private property within public view from any highway, public park or reservation any billboard or other advertising device, whether erected before August twenty-fifth, nineteen hundred and twenty, or not, which advertises or calls attention to any business, article, substance or any other thing, unless such billboard or device conforms to the rules and regulations and ordinances or by-laws established under section twenty-nine; provided, that this section shall not apply to signs or other devices erected and maintained in conformity with law and which advertise or indicate either the person occupying the premises in question or the business transacted thereon, or advertise the property itself or any part thereof as for sale or to let and which contain no other advertising matter or which are maintained on land owned by a person, firm, association or corporation engaged in the outdoor advertising business if owned by the same person, firm, association or corporation on January first, nineteen hundred and twenty-five, and if billboards or other devices were maintained thereon prior to August twenty-fifth, nineteen hundred and twenty, and have been maintained thereon from that time until January first, nineteen hundred and forty-five.