Slowly reading through the background material in Inferno, I came across this passage in the early history description of the Space Wolves.
The passage is interesting from a number of different facets. First, the Imperial was under existential threat very early in the initial portion of the Great Crusade. This was against the Rangdan, in a series of conflicts that would become known as the Rangdan Xenocides. Featuring losses that would not be seen again until the Heresy, the passage indicates that:
"...entire Space Marine Legions [REDACTED SECTION] lost to the Imperium…".
Now of course, the redacted section might just be "aspirants", or "companies", but there's no apostrophe here. Hence it is more likely that the redacted section would be along the lines "...including the primarch, XXX, were…".
Hence, are we seeing a huge clue as to the fate of one of the missing legions? Could it be that Legio II or XI were lost in the Rangdan Xenocides? From this passage, it looks somewhat likely by my reading of it!