Missing Legions?

Blood Ravens, One Of The Missing Legions?

I'm currently up to the last story in the Blood Ravens Omnibus, and during the morning read a piece on the Warhammer 40K groups wall about how the Ultramarines numbered 250,000, with the snide comment in The First Heretic saying the Ultramarines ranks swelled after the 2 missing Legions went missing. The other thing that got me thinking was how can we be sure that what we believe we know about the Blood Ravens is true? I've just got past the part where the 2 Librarians are performing the Rite of 100 Days and there is a lot of connections that the Blood Ravens have that a more modern Chapter could only dream of, and the fact that their identity seems to have been hidden by someone very high up amongst the Terran Administratum.

So based on that working I'm saying that the Blood Ravens rather than a loyal chapter of The Thousand Sons that came out of hiding after the Horus Heresy but are in fact the desendents of one of the 2 missing Legions, most likely the Terran 'Old Guard' of the said Legion that someone swept under the carpet till after heresy.