Over the years, I’ve read a lot about espionage, because of Matthew Clairmont’s life as a spy.
Recently, I took part in a conference organized by USC-Huntington Early Modern Studies Institute at the Huntington Library. One of the other participants was Jana Dambrogio, who is a researcher on “letterlocking,” leads a project at MIT to help understand this (almost lost) feature of early modern security, and is also part of an international project to come to terms with the trunk of unopened 17th century mail found in the Netherlands (http://brienne.org/unlockedbriennearchive/)
Letterlocking is a practice of making paper letters secure by using a combination of paper, string, and wax to “lock” a letter. There were different locks, all giving a different level of security from easy to break open to fiendishly difficult. All of those letters in the Blackfriars attics would have been letterlocked, by Matthew and his recipients and employers.
You can learn more about letterlocking from Jana’s website http://www.janadambrogio.com/letterlock/…, as well as the letterlocking videos on Vimeo (https://vimeo.com/letterlocking), which show you how to make your own letterlocks.