Ever since I've had my 87 RS, I've had my plate mounted a bit strange. There are two places for bolts in the back there, but the distance between them is two far apart to fit the plate, and even if it did fit, the plate would then block the trunk lock. For the time being, I have only one side bolted on, causing the plate to be a bit crooked, and every time I want to open the trunk, I have to take the plate off.

I'm guessing that there was a spring loaded hinge-like thing that used to bolt onto the threads there, then the plate would bolt onto that, but I don't know what to search for.

Am I correct, and what might it be called?

 

There is a hinged mount for the license plate that swings down for access to the hatch lock. Try locating one at a local salvage yard.