It is rather hard to express tone without using smile faces and !!!!. It can be a bit tricky.
Well now I know when MB started using electronic speedometers. Now I am assuming you mean they started using the elctronic ones in the W126 chassis, model year '81. My car is an '80 and it definetly has a cable. Now that I think about it, my father owns an '85 W123, and it has a cable. Maybe they first came out with it in the W126 then when they replaced the W123 with the W124, they switched for that class/size car also.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member