I've noticed that the NYPD uses some sort of custom fleet license plate such as that seen below.
I was able to find this reference that states:
No person shall operate, drive or park a motor vehicle on the public highways of this state unless such vehicle shall have a distinctive number assigned to it by the commissioner and a set of number plates issued by the commissioner [...]
As the license plate matches the identifier on the side of the vehicle, I'm guessing that this is an NYPD-created identifier (and an NYPD-created plate). Are these plates simply special ones issued by the commissioner? If not, how is this practice in accordance with state law?