Interesting stuff there,, the caveat is of course we can't prove the source of any of the letter's but several of them are unique and IMO may be from the hijacker or an associate.
For the coded letter, the FBI files note that they also have the unique code for the Seattle letter but they don't reveal it. The codes are to uniquely identify the sender later, to claim credit or extort money, if not the real Cooper the only reason a hoaxer would use unique codes would be to extort money somehow.. but communication never continued for some reason. Similar to Gunther, Cooper/hoaxer dropped off..
The codes had to be something the sender would know or remember, not an intentional imbedding of their identification. I just found an obscure match to the first code, it could be random but the odds are massive, the second one looks like an MGRS code which matches a location in Laos which is close phonetically to Los Angeles.. a long shot but still working on that one.
The hand written letter(s) about Cooper dying was the one I got un-redacted and it is amazing.. I believe it was written by the "Carla" who contacted Gunther.. either Cooper/associate or the hoaxers.. that letter matches the tone and purpose of Carla's communication with Gunther. To claim Cooper was dead and he was really a good guy.. that doesn't sound like something a hoaxer would write, it sounds like a close advocate for the real Cooper.
The Jacksonville letter is one that I think is very interesting as well and might be Cooper, I found some things in the letter that are very interesting.
I can't reveal the stuff I found yet.. I am trying to put together a comprehensive report.