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Early cars had mechanical speedos, driven by a flexible cable from a special gear on the back of the gearbox tapping the output shaft.
Later cars omitted this mechanical take off and instead used a special diff crown wheel with I think holes (like on the flywheel for the Crank Position Sensor) and a magnetic sensor. This was linked electrically to a special speedo head.
Your problem is you have a later gearbox with no cable take off provision.
I have heard that it is possible to dismantle the gearbox and substitute the rear end from a model that does have the take off. I don't know whether that is true.
Alternatively you need to put in a later type axle, and wire it up to a later type speedo. I don't know whether you can swap the internal crown wheel and use a later cover with the sensor mounting, or whether you need an entire new axle.
That would be easier, if you have one.
I have read warnings that all sensors are not necessarily compatible with all (electronic) speedos. So it would be safest to get both from the same car.
Finally, I have myself recently discovered that the wiring and sensor connector pins are not consistently wired the same way. I swapped in a known good sensor from another car and it didn't work. Close inspection showed the wiring orientation was reversed, so the sensor was trying to read backwards. I had to cut and rejoin the wires, ignoring the colour code.
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