This unit is a bit older than the ones I collect. I did some research on it and found it is actually a Radiola 18, RCA having made many of Brunswick's early offerings. Without pictures it is difficult to tell exactly what the component is that is in question. From the schematic diagram, which only shows the in cabinet wiring, the antenna input appears to be fed to the radio's circuitry through a variable resistor.
"Any sufficiently advanced technology is indistinguishable from magic."
— Arthur C. Clarke