Awk,
What is your vin number for the 280SL?
Should be 113044 with either 10 or 12?
Which one is yours?
Your car is a manual so it should be a 10, automatics are 12.
Might be a leftover from a conversion from automatic to manual as this wire plug could be the kickdown switch.
I say this, because the rubber grommet is backwards, meaning the plug is outside the car, not inside.
Other way to verify is by checking the colors of the wires.
Walter