How about some pics. I do not recognize what you are talking about in my 81 300SD. The sliding jaw fits inside a metal rail and forms a pivot for the regulator arm. As the arm moves the sliding jaw slides in the metal rail and raises the window.

As far as I remember you just remove about four or five bolts (or are they nuts?) from the regulator and motor and they both come out together. Seems like the metal rail stays attached to the bottom of the window and after replacing you slide the new sliding jaw into the end of the window bottom rail (has been a long time since I have done the job). Do not remember what those bolts/nuts screw into but believe they are a part of the door or door frame. Do not remember plastic bushings.

So need some pics.