It shall be the duty of the Purchasing Agent to maintain a detailed record of all automotive equipment owned by this city and requiring state registration. The Purchasing Agent shall obtain and assign state registration plates to each unit of automotive equipment owned by this city. Whenever the character of the services performed requires the operation of motor vehicles with state registration plates other than exempt, which are used in lieu of such exempt plates, the exempt plates so replaced shall be turned over to the Purchasing Agent, who shall be accountable to the Division of Motor Vehicles of the State of California for such exempt plates. Such license plates as may be used in lieu of exempt plates shall be obtained upon written application to the Division of Motor Vehicles by the head of the department to which the automotive equipment has been assigned. Such application shall be forwarded to the Purchasing Agent who shall transmit same to the Division or Motor Vehicles, together with such additional information as may be required. The Department of Water and Power and the Harbor Department are hereby exempted from the provisions of this section.