I posted a request on the Garry's Mod Dot Org forums for a custom in-game Sentry Manual prop based on the cover of the manual that comes with Team Fortress 2 in PDF form. I thought it odd that players can read the Sentry Manual, but the characters had no way to, at the time.
So a guy called Micropro made this manual for me. It comes in four different colors, including green and yellow, in case you're using the green or yellow TF2 skins.
It wound up being even less important in the final story than it was in the first draft, but it's in there as a nice little easter egg. Also, I made the above image as a "thank you".
Another manual was also made by SBMPHysteria, but Micropro put a little more effort into his so it was the one I ended up using.