Looking at an illustration of that distributor cap on a parts website it sounds like the distributor cap was installed 180 degrees off so the plug wires didn't match the cylinder numbers on the distributor cap. Presuming the distributor cap can be installed correctly or 180 degrees off, all you needed to do was remove the ignition wires and turn the distributor cap 180 degrees, not fool with the distributor itself.

It's likely a MB dealer tech who works on those engines regularly installed the distributor cap incorrectly. But since he knows how the ignition wires attach to the distributor cap he doesn't need to rely on the terminal markings like a DIY'er would.

__________________
Fred Hoelzle