Something new I found with a Google search of “blackmesasource” and “3914” as a last ditch effort before heading to bed–looks like a website referencing post 3914 led to an employment letter teaser that was released a while ago:
https://www.blackmesasource.com/bmrf-employment-letter.pdf
Note the “tb” at the bottom. Look familiar? I’m starting to think we need to look more outside the box on this one guys–those codes are pointing to things we aren’t going to find in the game.
EDIT: Something I did notice is that if you look up “Gordon Freeman” in this site’s wiki it mentions the letter. I guess it could just be coincidence that I found this using the search strings that I did, but no one has mentioned this yet. It WAS released as a teaser during the countdown leading to the release, so it’s quite possible it isn’t related. In fact, the more I look at it (aside from the signature of “tb”), it just looks like a copy of the original letter, save a few BM-related changes. Hmm . . .