Originally Posted by A. Rosich

The two tone horn was optional equipment for W123s, W126s, W201s, early W124s and I believe early W140s.

And by two tone I mean selectable town or higway tones (as mentioned before by 79Mercy), as most Benzes do come fitted with a two tone horn standard (high and low tones that sound together).

Basically, the town/city horn was a single or double electric unit and the highway/country horn was air operated (which sounded much more louder).