Production used letter from the film The Green Knight. In the film, the Green Knight proposes his challenge by presenting the King and Queen with a letter such as this. It is unclear how this particular letter would have been used, as production made several. It has signs of production use, including wax remnants from the Green Knight's personal wax seal.
It seems plausible that multiple takes of the seal breaking shot were done, which would explain the condition of the seal.