Secret Ending and Letter Locations

In Chapter 19 you will come across a computer that requires a password. Finding this password entails tracking down a series of letters hidden across the various chapters of the game. Finding these letters and accessing the computer will unlock a secret ending that reveals hidden story details.

The location of these letters are detailed in this video (thanks to TheFourthSurvivor):

<youtube>https://www.youtube.com/watch?v=2oISmZH6y04</youtube>

The details of the secret ending likewise are viewable (SPOILER WARNING) here (thanks to DiscoNachoPiggy):

<youtube>https://www.youtube.com/watch?v=ZFTUeT0jqfo</youtube>

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