On a 521, when the combo meter get ignition power, it applied 12 volts to the oil light, and the (IGN) light, ans well as supplying power to the gauge voltage regulator.
If the engine is not running, the oil pressure switch grounds the oil light, it comes on. The alternator also grounds the (IGN) light, and it comes on.
Grounding the sender wire for either the fuel gauge, or the temp gauge, causes the respective meter to read full.
I believe a 510 acts similar to this.