Hi, it's me again.
You may be interested in these radio interference suppressors I've accumulated over the years. They were made by Belling-Lee, Aerialite & Dubilier plus an anonymous 2pin example. Most of them released smoke when I plugged them in which, I assumed, was from the ancient duff caps. This was long before I learned that caps could fail
The Belling-Lee one has two wander sockets marked "Earth" & "Set Earth" and had two plugs marked likewise. I may still have them somewhere. The Dubilier one is made of rubber.
Cheers, Pete