1857 GB Cover And Invisible Ink Question

I have another from GB that piqued my interest. It doesn't have any writing inside, but when held to certain light you can see reverse writing on the outside of the cover which is dyed in a strange way. I read about "invisible ink" covers that spies would occasionally use during the 1800's to send hidden messages. Could this be something like that?
The inside flap reads "Hoggins dead".

It is much more likely that you have only 1/2 of the mail piece and that the actual "letter" portion has been cut off by a previous owner for privacy reasons or to make cover storage less bulky or to sell the interior letter for an interesting autograph, etc.

The "8 April 1857 / Hoggins ...." writing in the last image is docketing added by the recipient as they refolded the letter for filing purposes.

More likely "Hoggins deal", I think.

I liked the invisible ink, spies and death story much better. It added to the value.

No conspiracy. No invisible ink. Just an ordinary mourning cover. Or as GeoffHa wrote an even more ordinary cover with a deal. The latter most likely, as mourning mail, usually, had a black frame.

NSK is correct - it's not a mourning cover, which have black borders of varying widths.

A typical solicitors letter from the period, which are in abundance. Even I have a few

Potentially a decent stamp to find. Die 2 Alphabet 3? It's perforated 14. Unknown watermark. On an optimistic day it could even be one of the transitional issues on yellowish to cream paper. Good postmark date for that stamp. Better image needed.

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.

Scotzm your knowledge and instinct for penny reds is outstanding
I just started learning about them this month, so I have very little knowledge on what might make one unique. But I took a mental note of this one because of the creamy paper type. It's amazing you could see that from a small image.


They're very easy (and cheap) to come by. So what I like to do is buy a ton of them and absorb whatever I can, however long I can, before doing any scholarly research. That way there isn't any voices in my head pulling me one way or another when examining them.
And thanks for the mourning cover explanation! I now know what the black border means. Here are some others I just got. The ones in the France cover look more crisp, so I'm assuming they're a later printing-