The speedo cable goes into the clamp #112 270 01 13. That housing has an O-ring that can leak and is #000 997 68 45. What happened to me was that when my W111 car sat for two weeks or longer, the trans fluid would drain out of the fluid coupler and fill the pan. This level would go above the housing O-ring and would leak. I replaced that O-ring and the leaked stopped. I also noticed that the speedo clamp on the housing was cracked and probably due to over tightening. I have a replacement but never installed it.
There is also a plate with a gasket (#112 274 01 80) that allows you to remove the gear drive in the trans from the speedo cable. That is attached to the trans case and can also leak. All of these part numbers come from EPC or whatever Mercedes calls it now. If you do not have it, it is worth the cost or use Niemoller for their drawings. I hope this helps.