Hi Crash,
Glad you enjoyed the extras!
And now the truth...those 'letters' were faked for Erwin K. Robert's introduction as a joke on how often 'lost cities' have been used in movies and literature. It looks like I did a pretty good job on them. I used Google to find the stamps and logos and put it together in Photoshop. It was a lot of fun to pull them off.
It was done in the same sense of whimsy as the DVDVerdict review of the Lost City serial on pg79 of the collection which has a fake conversation between William Boyd and his agent.
Stay well,
-Yoc