Notwithstanding any other provision of law, it is lawful for any vehicle regularly used as a rural mail delivery vehicle to display two simultaneously flashing amber lamps mounted on top of such vehicle while it is being used to deliver mail. The light assembly must consist of two lamps mounted on top of the vehicle with one lamp being as near as is practicable to each side of the vehicle, displaying an amber light not less than four inches [10.16 centimeters] in diameter and visible under normal atmospheric conditions for a distance of at least five hundred feet [152.4 meters] to the front and to the rear of such vehicle. The lamp assembly must include a sign at least seven inches [17.78 centimeters] in height containing the words “U.S. MAIL” in black letters not less than four inches [10.16 centimeters] in height and of not less than three-quarters of an inch [1.905 centimeters] in width of stroke, upon a white background. The sign must be constructed so as to permit folding down out of the line of vision when not in use. The lamps must be equipped with a device to cause them to flash on and off, and such lamps must be so wired as to cause both lamps to flash simultaneously. In lieu of the light assembly permitted by this section, a vehicle may display one revolving amber light placed on top of the vehicle and accompanied by a sign placed on the rear of the vehicle and containing the words “U.S. MAIL”. The light and sign must comply with the requirements applicable to the amber    lights and sign used with a light assembly permitted by this section. Amber lights permitted by this section may only be operated for the purpose of discharging official duties and must not be in operation except during the actual performance of duty delivering mail.