Hi Frank,
You can use the front quarter door window turn knobs from the W110/111 fintail series which look identical externally.
The center arrangement of the knob needs to be "adapted" to the rod mechanism of the SL seat's backrest (you will see what I mean once you have it in your hands).
It's available from new from your local MB dealer, any 110 723 -xxxx number (which I currently forgot).
Not too expensive though (around $$ 10 or so).
Good luck!
Achim
(Magdeburg, Germany)