As Stephen indicated, the car you are referring to is a 6500 series flat. It was modified by the D&S for use in loading and unloading a CAT.
The coupler on that end looks like it was home made, if I remember correctly. It has quite a long drawbar. The original pocket could not be used. I spent a few minutes in June looking at that car in Rockwood, where it was parked on a siding. They did a nice job on the conversion.