Back-up rings have no intended sealing function. Instead, as their name indicates, they are protective and supporting elements made from extrusion-resistant materials which generally have a rectangular cross-section. They are installed in a groove together with an elastomer sealing element, preferably with a corresponding O-ring in static applications.
January 1946: The origins of Heathrow Airport
Interesting the first flight mentioned in your original 1946 article was a Lancastrian "Stardust" which became famous when it disappeared on...