I note you have a proper ground connection to sender. Many vehicles of that era had OE only the one sender connection, relying on tank to chassis grounding through fixings to complete the circuit.

With the moving iron gauge you almost certainly have a check on the instrument itself is achieved by simply removing the connection to sender. With ignition on the gauge should read instantly beyond full, and similarly with same removed wire touched to tank, the gauge should read below empty. This latter check also proves the aforementioned tank to chassis connectivity.

Richard

2018 Roadster 3.7
1966 Land Rover S2a 88
2024 Royal Enfield Guerrilla 450
1945 Guzzi Airone