Yep, the backseat seatbelts are the same for 83.5-85 w123/w201. The fronts are too kind sometimes depending on the w201. The 86 and up 190e's with airbags dont work if my memory is correct, but the 84-85 work.

Not sure about the older styles, does anybody have a w115/w116 to compare? Those are getting hard to find at the junkyards these days (at least here).