Just to reiterate, these are addons already in the server that have been removed from a previous content pack. These will not need to be added to the server, but are already in the server and spawnable. Used in many dupes around the server. I have come into contact with some Jedi that found spawned props such as jedi books spawn as errors. Clearly there is a mismatch between the current content pack and the server's addons.
I actually traced back errors logged in my client's console from the server when I encountered a missing model, these are the addons I believe the server has. Of course, from the perspective of someone with access to the server's filesystem and mounted addon list, one could find out which of these is missing and add them to the content pack, but these are the ones I have narrowed it down to.