Speedometer sensor - HELP

Hi all
Just getting my '86 Atmo back together.
Engine rebuild done, gearbox refurbish done, extractors fitted, and Quick Fuel 450 Black Diamond 4-barrel carby fitted on modified inlet manifold. Will post build pictures and details in the next couple of week.
However, we do have one problem.
We appear to have what looks like two speed sensors up at the gearbox end. I reckon there should only be one.
Questions - does anybody have the colours of the wires for the speed sensor (3 wires) and any thoughts about why we might have two similar looking devices?
Any diagrams or pictures would be appreciated.
Thanks
Graeme

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Thanks Clee
Car is going - running really well. Just adapting air-cleaner and will be going for a 500 km jaunt this weekend.
As a matter of interest, the wiring for the speedo sensor comes out of the big plug and socket on the bulkhead in front of the engine.
The other sensor, which looks the same, has its three wires disappearing back into the look that serves the alternator.
Any thoughts about that would be appreciated!
Cheers
Graeme