Any idea how they are mounted from the factory?I guess the long black metal strip goes behind the bumper and using 3 bolts you fix the license plate support.I think I am missing the side bolts from the metal strip,but I have no idea if they are the same as the middle one.

 

The raised outer section should be at the top - but that is all I know as my own one is now secured by four self-tapping screws into the bumper. It's probably been on and off a few times in 25 years, so the original fittings have long since gone!!