Where did the information about the possible British 20-inch gun come from?
Short answerThe reference was found in the Tyne and Wear archives.
NotesThe information is attributed to a search in the Tyne and Wear archives. Other archives mentioned as useful for naval research include the Wirral, Glasgow, and the Churchill Archives Centre at Cambridge. Naming the repository gives the uncertain gun claim a point of provenance without strengthening the underlying evidence.