Post your chassis no and someone will come up with an answer. Its possible you may not have the original engine in the car? so an engine number might help the actual no not the log book one.
Like you I have not been successful in tracking down a part no---- found several candidates----- the best being Bosch 0280 130 201 but far from certain. I think your best bet is to go to your local Bosch [ not Mercedes] agent and try and get the part there. One good thing is that many fuel injected cars around that time used similar thermo time switches e.g. FORD and VW and BMW so they well be still available.