Underlying article in Nature: https://www.nature.com/articles/s41467-021-21326-w

The letters being opened are “locked” with a technique that was obviated by sealed envelopes, but was prevalent in the Renaissance and Middle Ages.

It’s super intricate, though! Might be a fun thing to try for your next mailed letter :)

https://en.m.wikipedia.org/wiki/Letterlocking