There are a lot of wires of many different colours behind the dash and perhaps over the years previous owners have made some mods and changes to add to the confusion.
Although a colour coded wiring diagram can help they do not always explain the wiring involved on the earthing of a circuit.
As in this example the wiring diagram would suggest a direct connection to earth for each gauge bulb but in fact the earth for each bulb is taken back within the wiring loom to a common earthing point on the back of the tacho to one of its fastening screws.
Bulb holder showing centre power connection and outer earth connection.
The alternative found on chrome bumper and early rubber bumper MGBs is a wired loop fixed under the gauge clamp knurled nuts which is taken to earth.