According to this document
http://amtrans.nl/mercedes.pdf, my available options to find an electric speedometer for a 722.4xx transmission, would be the following:
The 190s are all mechanical from what I can tell. Most of the 300 diesels are mechanical from what I can tell, the possible exception being the 300SD?? Looking at clusters on dBay to see what have mechanical connections vs electrical, I have deduced that the following appear to provide the best possibility of finding a rear housing with an electrical speedometer that will bolt to my 722.416
C220 94-96
C280 94-96
300SE 89-91
300SEL 88-91
Any other insight in where to look?