Thanks for catching! I had totally missed them discovering the drawer (I've updated the article to include this detail). Part of the trouble, though, is that the letter wouldn't miraculously appear where it hadn't been before. An important detail of Outlander is that things don't actually change like in Back to the Future - if Roger had left the letter in the drawer in 1739, it would have always been there. If it was, how did it stay undisturbed for so long?

The book followed that rule by revealing that Roger's letter was fixed to the top of the drawer unseen. Unfortunately, the TV show had to speed through this and simplify, breaking the time-travel rules as it did. I find this to be a bit of a bummer!