Section 7I. Special purpose motor vehicles and trailers, the property of and registered to charitable corporations, specifically assigned to the emergency disaster services of those organizations, and used for the benefit of firemen, policemen, civil defense workers and victims of fires and disasters, shall be furnished, without charge, by the registrar at his office, a number plate of the type and design furnished to ambulances, fire engines and fire apparatus. Such emergency disaster service vehicles may be equipped with sirens or other audible warning devices and with visible warning devices as provided in section seven E. Such audible and visible warning devices may be used by the vehicle operator only when responding to an official alarm of fire or disaster and at no other time. Every such emergency disaster service motor vehicle shall be marked, on a part of the vehicle not readily removable, and in a conspicuous place, with the insignia of the corporation and with words identifying the vehicle as an emergency disaster service unit.