Given the vintage of the car and the previous owners it is bound to have had....

It looks like someone needed an alternator but didn't want to spend the money for a new one. But they had a generator and a voltage regulator and found the the generator would fit. Hence the addition of the regulator needed to make the generator work with the car.

Then, someone else owned the car and decided to put the thing back right and just blew off removing the regulator. Or perhaps the first 'solution' didn't work so well and melted down the plug. Then they owner would have to have sprung for an alternator anyway.

In any case this is not something that came that way from the factory. I mean, it is a Mercedes. You can order almost anything you want and maybe someone was too old school to want one of them new fangled alternator thingies.

In any case an interesting question. The good part is you don't need to worry about it.

And I bet if you removed it and found a Bosch part number on it you would find it came from a Volkswagen.