What is the large silver "button" used to fasten rear Japanese license plate on vehicles called? One of the two screws is larger and has a prefectural seal affixed to it. I want to determine its purpose and name.

This seal is called .

From JAF (the Japan Automobile Federation):

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> (Roughly, _The purpose of the seal is to block the detaching of the licence plate as well as being a preventative measure against motor vehicle theft._ )