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.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section