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Elise S2 Non ABS
The Rover S2 uses 3 identical bearing packs without sensor and 1 pack with a sensor for the ECU (which then relays it to the speedo) on one of the rear wheels.
Bearing pack without sensor: A117D6002F Bearing pack with sensor: A117D6005F
Elise S2 ABS
Use 4 identical bearing packs, ALL with speed pulse for the ABS signal (which in turn feeds that to the ECU)