In every place of the World car owners should have special car license plates numbers. Those plate numbers can be made from metall or plactic. The main goal of numbers is the identifying of the vehicle. So car drivers should be carefull about those plates and don't leave of forget them.
That is the law and these items should be obligatory set there. They can be of different shape and made from various materials. It depends on the country transport regulations. Of example, in the USA we have square metal or plastic boards.
If you have found or lost a license plate that begins on J6, then you can submit your request here.