[RGK20m3]

On my 38-690, the link between the black and ground terminals is a soldered wire inside the chassis; I believe this to be factory, as it is a continuous cloth covered wire from the 2 lugs to the RF sub chassis tab. From the tab there is an identical wire bonding the RF sub chassis to the main chassis.
Of course, it could have been changed at any point in the last 85 years. I do see how the antenna coil could provide a balanced input, but seems would create an issue as there is no ground reference on the circuit. Most I have seen have the coil center tapped.
Per my above post, at least one of the Philco dipole kits has a “transformer” at the mid point, and the literature for that kit (Philco Library), instructs an additional ground to this transformer if there are interference issues. The photo link that I posted shows what appears to be a balun/ transformer included in the kit. Perhaps actually an balanced to balanced matching transformer?
Not trying to argue, just trying to understand. Incidentally, that schematic you provided does not match the 38-690 schematic.