The plastic piece is called the sliding jaw. It is designed to break when the regulator arm gets too much tension. I have replaced about five of them in my '81 300SD over some 35+ years. I just bought a new type sliding jaw made by a forum member Diseasel300; will report back as soon as I install it.

The problem with the original sliding jaw is that you have to drill out a rivet which holds it in place. The new one does not have a rivet.