I'm at a loss at to what piece you are referring to, just the sheet metal edge? Yes there is a plastic trim piece that basically covers that up, it's attached to the package tray. The package tray is secured with two 90* angled sheet metal pieces from the front.
Ok, that piece of sheet metal you are pointing to doesn't actually have anything secured to it. The plastic trim that will cover that up so it can't be seen is actually attached to the back of the package tray, that platic piece comes with a new package tray if you order one. Other than that the only other thing that secures the package tray are the two metal angled pieces I explained and the rear seat back itself. The screw holes for the little angled pieces should be just to the inside of each of the seat back hooks.
Also not sure what year your Charger is but I'd recommend picking up Project Charger by Larry Lyles. He goes step by step on the restoration of a 1970 Charger and has a lot of good reference pics. I used it on my Roadrunner and helped a bunch.