the rear plastic clip will hold the DHC in place when the front screw is installed (through the plastic collar to prevent the screw from damaging the paint). I've had examples of not perfectly matching DHC where they bind and won't stay in place properly w/o the lock cylinder installed, that's how i'm sure it was a similar thing with your case.. the lock cylinder when bolted in will hold the DHC; the plastic clip is just for during install.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)