Elheru Aran wrote:
You may be thinking about the Blood Ravens, who speak of a 'unknown Primarch' on occasion and... it actually hasn't been said outright where they come from, but many think they've got Thousand Sons ancestry, given their large numbers of Librarians. I'm not sure about the Raven Guard, but as current GW canon stands, the Raven Guard are First Founding, and there are only two Unknown Primarchs/Legions.
The book A Thousand Sons has some passages that all but scream "The Blood Ravens are descended from The Thousand Sons." Besides all of the corvus imagery, we find out that Magnus sent the fleet (with some Thousand Sons aboard) off with secret orders before the battle, and a couple of psykers make statements about "A Raven of Blood" which is "seeking understanding and redemption." I think that's about as clear-cut an answer as we are ever going to get.
In an old short story, Hell in a Bottle, there is a mention of the Primarch Rubinek of the Iron Hearts chapter of space marines. When the story was reprinted later, GW changed Rubinek to a Chapter Master. Make of that what you will.