So, in the Roguelike celebration presentation there was a mention that it is hard to figure out how to make sure that a player will be able to find the link between a group of bandits(or an assassin) and their contractor sensible without being too straightforward.
So I was thinking, there's like plenty of stuff you can do with the paper. For example, you can use only paper/parchment that the villain's home site gov has access too. Other things you could do are for example, have a given site government maybe water mark their paper. This'd be a bit more like a spy thriller, where you'd have like 'oh, this paper is office stationary from this or that company'. Watermarking is within the cutoff period,
an Italian paper maker using a little metal wire during pressing for the watermarks in 1282. An investigative adventurer would then know to hit up that site government to continue their search.
Another idea would be to have villains have their own symbols, like site govs have, and have them own little
signets/sealing stamps that they use to seal/sign their evil schemes with. Players can then find letters with these symbols and if they find a person who is carying a signet on their person, they are probably the author. This one's nice because it is something that the computer can check so the player can appropriately accuse them of organizing villainous schemes.
Seals have been made with clay(in the game), sealing wax, which is made from beewax(in the game) and tree resin(not in the game), and in Asia stamp signatures fill a similar purpose. These would require an ink or dye at the very least, so I guess we'd mostly see clay seals.
A third idea, is reusing written-on materials. This one is necessary because you cannot watermark parchment as far as I know. However, you can
reuse it. Scribes would often reuse parchment by scraping away the ink. Given there's letters in the game, maybe a villain can scrape away contents from a letter they own, and the player might be able to just see a significant word, like the name of the writer or addressee. Similarly maybe villains reuse an old quire and then the player might be able to see 'on the back of this note is an excerpt of a manual, it mentions the town rhymevaults.'.
The tricky bit with the reused letter stuff is that you should be able to ask people if they know people living at a given site, so that you can try to deduce who the letter was sent to, to begin with.
Anyhow, those were some of the ideas I had, maybe other people know more fun spy/detective novel tropes that can be used to link villains and the assassins/bandits they order around?