I've looked at a bunch of photos of rear euro 123 bumpers. It appears the chrome part of the bumper, without the bumper trim.. sticks out about 1" - 2" past the rear tail lights. When I put mine flush against the body, it is recessed behind the level of the tail light, or just even with it.

In a few photos I've seen, I can see a part of mounting plate that sits below the bumper on each side.. this is the part I think I am missing.