This item is a "POLAR RCC"
interstage coupling device. Usually, to acheve a decent measure
of gain at audio frequencies one used a step-up transformer of
say 3:1 ratio or more, but transformers such as this were relatively
expensive and not too rubust. The alternative was a mysterious
part such as this which is simply a resistor for the anode circuit
of the driving valve and a coupling capacitor which connects
to the grid of the following valve. It was known as an RCC or
This old part has 6.7nF between terminals
marked "A" and "G" and 0.144nF from anode
to HT. Anode to HT measures 4.5Mohm and the GB terminal seems
to be open circuit. Unfortunately this type of component would
work when correctly wired up, but in the hands of an experimenter
this example seems to have died.
The coil which is visible might be resistance
wire for the grid leak, but measures open circuit.
Below is the cover with labelling
for the RCC. The experimenter wasn't helped when glue failed
and the cover carrying the markings twisted around.