Docketing added by the recipient makes total sense. The writing style is very different! Thank you John Becker.
My main question was about the backwards writing seen faintly on the front of the cover.


These were my thoughts:
-It is only visible on the outside of the paper. This means that it couldn't have come from the contents inside, otherwise the ink would be visible from the inside of the cover as well.
-The writing is seen strongest from the right side, then visibly fades to the left, but uninterrupted by creases. This means that it was added to the paper before the paper was folded.
-The writing is backwards, therefore it could be writing from a separate document that was then pressed against this one to produce a mirrored effect.
I did not think this was plausible, since both top and bottom margins of the writing is clearest from right to left. Ink will always be most wet at the bottom margin, so if this cover paper was pressed on top of a fresh document, the ink should be more visible at the bottom right to left margins then fade to the top.
My first theory was invisible ink. If the outside of the cover faces 150 years of various environmental changes, it would make sense that the outside would start to show the ink while the inside would remain clean. So I did a quick Google search and found out about 19th century spies and whatnot, then posted about it here to see if anyone knows a safe way to test the inside of the cover for a potential visible ink usage.
Maybe I am missing something very obvious! So I thank you for the comments.