Speedo options after swapping diff in 1981 300TD

[240dddd]

I just changed the differential in my 1981 300TD. I swapped in a 2.88 diff from an '85 sedan. The original diff had a sensor installed for ABS, although the ABS had never worked for me since the front wheel sensors had been removed by a PO. The newer 2.88 diff does not have a hole for the ABS sensor.

After finishing the diff swap and removing the ABS sensor my speedometer stopped working. It seems the 1981 300TD did indeed have an electronic speedo and I suspect it was actually driven by the sensor in the diff.

Assuming I am correct about the diff driven speedo (please let me know if I am wrong) I need to figure out what the other options are possible for a speedometer.

This car is a bit of a Frankenstein. A PO added an OM617 turbo engine and a newer transmission. The transmission in the car is a 722.315 and has the sensor for a tachometer. I know this because I was able to get a tach working using the wires coming off of the transmission. I think this means it was probably a transmission from a later model w123, i.e. 1984 or 85.

My question is whether my electronic speedo can driven off of a sensor on the 722.315 transmission? Or is my best bet to track down a cable driven speedo?

interestingly the speedo does have the circular connection that screws onto the back of the speedomter which I thought was the speedo cable?