There must be one gazzillion threads discussing the basis for this subject.
M.B. set the timing of the M103's depending on the market and quality of the Gasoline.
You need to look in the owners manual for your particular vehicle and it will state the Octane to be used.

The famous R16 resistor sets the timing and hence the Octane rating to be used. Just do a search for R16 resistor.
here's an image of the resistor:


Some markets had variable fuel so MB included a resistor wheel - you could set the timing yourself depending on the available fuel octane rating.
here's an image of the wheel.