This is a basic bracket allowing people to install their ghost shadow lights on the back of their backbox instead of drilling permanent large diameter holes into the top of the backbox.
You can either screw in the bracket to either side of the back of the backbox or one in the centre and instead of screws you could use double sided tape in the home environment.
There are 2 different brackets:
23mm - This is for the ghost shadow lights that are supplied with a 23mm hole saw, these typically have a red line and locking hex screw and a little longer in the shaft
19mm - This is for the ghost shadow lights that are supplied with a 19mm hole saw, these typically have all black round locking nuts.
check with the ebay suppliers of what hole saw they provide so you can check on the correct bracket to purchase.
Search "swinksGSL" for the different brackets at Shapeways
https://www.shapeways.com/search?q=swinksgsl
The hole size in the bracket for this one is 20mm and the alternative design is 24mm.
Here are some links for various ghost shadow lights - search Ghost Shadow Light on ebay and there are all sorts of options like Mustang, Ford, Chevy, Burning Skulls (No Fear), Batman, X-Men etc. This bracket is a little more expensive as it is longer if both directions and wider.