I’m quite sure there is no coating, only patina on my Charles Smith key. It hasn’t even been buffed or polished. It is easier to see that in real life, I can understand how it looks like it could be a coating in the photographs. I really wish I knew more of the history behind it! In future, I to take better pictures of it when I have access to a quality camera.
I think ink the engravings on your key are justification enough to say it’s a prison key.
…Mark