I just went out and checked out my bumper, and may I say..damn is it cold. Anyways, it does appear that it is a angled piece of metal that is welded in place. I gather what Mercedes did was take the bumper for a W123 sedan, notch it out for the lights, then add the angled bracket. You'll notice that the area around the light it'self has a rubber seal on it. I am guessing that this to cover up the cut in the bumper. So you bought a bumper that has the notches cut out, but no means of attaching the lights themselves?. I would ask this vendor how exactly you're supposed to attach backup lights to the bumper he/she sold you.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member