Every truck unit and unit of automotive equipment other than passenger motor vehicles owned by this City shall at all times have painted or imprinted on each side, preferably on the cab doors, the official seal of this City not less than eight (8) inches in diameter. At the option of the department to which said motor truck or unit of automotive equipment has been assigned there may be painted or imprinted in letters two (2) inches in height, immediately below said seal the name of the department and immediately below such name there shall be painted or imprinted in digits one and one-half (1-1/2) inches in height, the automotive equipment number assigned by the Purchasing Agent to the truck or equipment unit. In the absence of the department name there shall be imprinted immediately below said seal the automotive equipment number assigned by the Purchasing Agent.