[Chas]

Terminals 4 & 2 create an antenna input coil of which one end is earth ground the other antenna. That makes the entire coil assembly a three circuit tuner.
Option is given to ground the chassis or not to earth, depends if the ground is a counterpoise, useful when no earth is nearby such as 2nd or higher floor of a building.
Being a regenerative set such an antenna input option helps to prevent hand capacity effects. Might look about the cabinet for some sort of connection to earth via chassis or other to the front foil design.