The stain on the far left, middle, in the 1sr image. It's mirrored on the 2nd image. I'm assuming it's a wax seal. There were also paper adhesive seals used at the time.
My new interest. https://mitpress.mit.edu/9780262049276/letterlocking/
https://youtube.com/@letterlocking?si=w2aLIpn8Mz6vSYDm