Have any of you gotten their hands on the new
Letters?
Regardless, here's an excerpt from the
Nerd of the Rings' review of the book, in which he elaborates on the substantial extension of the 'Letter 131', with some material that might be pertinent to this project:
https://youtu.be/O2tACra4AJw?t=533
To be fair, this letter is from the early '50s, I think - therefore, much of the material that Tolkien later wrote (some of which, like the
Athrabeth, he explicitly wanted to include as an appendix to the
Silmarillion) don't appear in this outline, naturally.