Any person, firm, or corporation in this state engaged in the business of equipping, modifying, repairing, or detailing motor vehicles or trailers which are not registered and which are not owned by such person, firm, or corporation shall make an application to the department for a registration certificate and one license plate. Such application shall be accompanied by a fee of thirty dollars. Additional pairs of certificates and license plates may be procured for a fee of thirty dollars each. Such license plates shall be designed by the department and shall bear a mark and be serially numbered so as to be distinguished from each other. Such license plates may be used solely for the purpose of equipping, modifying, repairing, detailing, and delivering such motor vehicles or trailers. Upon demand of proper authorities, the operator of such motor vehicle shall present a written statement from the owner authorizing operation of such motor vehicle or towing such trailer.