The plastic item you describe does not bolt on, it has hooks that fit in slots in the bumper. There are two of them and they slide toward each other.
I found when trying to get the strip back on, I would knock off the other side, so with the rubber strip completely off the car, I got both of those plastic "bumper joint covers" on the car, secured them together by using kite string, then slide one end in the corner, bang the rubber strip on half way, slot the other corner in and simply pound it on.
Pics here:
W123 rear bumper rubber trim