Motor vehicle identification is displayed as follows:

(a) For most Government motor vehicles, preferably on the official U.S. Government license plate. Some Government motor vehicles may display motor vehicle identification on a decal in the rear window, or centered on both front doors if the vehicle is without a rear window, or where identification on the rear window would not be easily seen.

(b) For trailers, on both sides of the front quarter of the trailer in a conspicuous location.