Thanks
i am guessing from the lack of information around the internet that no one has come across this piece.
i have tried that but it still wasn't there
Not sure where to go to get it or if you can even get it through normal means, but the code for it is 1921d8. So if you are using the PC version, then you can use console commands to get it. There are tons of items that are in Fallout 3 and New Vegas, but only available through console commands so it is possible that Fallout 4 has its own items that are only available through console commands. If you are using the Xbox One or PS4 version and the only way to get it is through console commands, then you have to wait for mods.
Not necessarily. The Strategy Guide starts development months before the game is released. Cait's harness could have originally been acquired through some mission, but for whatever reason it was cut after Prima talked to Bethesda. Also, content can be forgotten. So an art designer is told to create Cait's harness, but someone forgets to tell the mission designer to add it as a reward for a certain quest. If you have 50 different people working for 5 years on a game, then things will be added that only one person knows about and has forgotten that they added it. Just because something exists in the files, doesn't mean it exists in the game.
thats a good point. I hope its in there as it looks like a pistol shoulder holster from the photo. Well if anyone comes across it other than console commands on pc please let me know.