The one on the top rear of the head is connected to the gauge in the speedometer. The one at the front/water pump may be an electrical contact for an another gauge perhaps fitted under the dash. The one at the rear of the head works using a bourdon tube filled with ether. These are frequently broken when trying to remove the sender from the head. The tube doesn't like being twisted. That may be the reason for the extra gauge. Original gauge can be repaired by a specialist or bought as a replacement. Neither option is cheap. Again a reason for the alternate one. With an adaptor an electric gauge can be fitted to the original rear position on the head. Sender must match the gauge electrically.
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