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Original gauge... Likely a King-Seely system. I do not really understand it but what I do suggests the sender sends pulses to the gauge and bumps the indicator, the more pulses the higher the indicator goes (don't ask me how).
Aftermarket sender... (used by most aftermarket gauges) senders use a rheostat (variable resistor) to vary voltage and the gauges is basically a voltmeter.
King-Seely gauges will ocassionally, but not always kinda work with the aftermarket sender. Empty could show as empty, Full could show as full, but anything between will be random to that gauge and sender.
There are some great threads on the subject with 747 explanations of how the King-Seely system works. Search for them. (747 jet... way over my head).