On military staff cars there's a metal bracket, usually attached to the front bumper, that is used to display a special plate that serves to identify the VIP passenger in the car. Your plate appears to be one of these VIP plates; in this case, one for a U.S. Undersecretary of Defense; and in this particular case, a full General with the Defense Staff (denoted by the 4 stars).
Regular Army generals would have a plate bearing one to five stars on a plain background on it.
Good find.