hmm, where do you see it as Raventhiri?
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Ah, there it is atleast twice in the Field Manual.
If I had to guess (and I do, since I don't know) its possible that the "iri" might symbolize a pluralization of the name.
For instance, its the Raventhir name, but if you were talking about all of the Raventhir family trees/bloodlines, you might just call them the Raventhiri. Of course, this is just speculation and I can't really back it up in any way shape or form :)
As to why we know so little...the eras that the Raventhiri seem to be most prominent in are some of the least detailed for Battletech. Near the end of the First Succession War, late in the Third Succession War...these are eras where there hasn't been much of anything written.