I dont think that it was, maybe an option, but mercedes doesnt put the lsd in many cars, at least 80's vintage. You could jack the rear of the car up and spin one tire, if the other one doesnt spin then chances are you dont have a lsd.

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Bart
1987 300SDL-T
1985 190E 2.3-16
1978 240D
1957 220S(running one day, I hope!)