1.The book Fulgrim:The Palatine Phoenix is set in Early Great Crusade,because only 8 Primarchs have been found at that time(including Fulgrim)-and Fulgrim mentions that 6 of his brothers were against him and that the seventh was "master of the Second"-its all in the book.So it's exactly the Second Primarch.Also the thing confirming that it's Early Great Crusade is that Emperors Children are not up to their full strength yet and they are still having that virus.And Eleventh is not found yet-the next part I wrote confirms it.2.On the page with Primarchs there is an official list of their rediscovery given by Laurie Goulding.In it there is one of the Missing Primarchs and he is found right after the Russ,so he is being the third Primarch to be found.Also confirmation for this is in book "Wolfbane",where Russ confirms it by saying:"Firstly we found Horus,then me,then the third brother and we all know how tragic was his fate".So he confirms that one of the Missing Primarchs was found third.And Fulgrim confirms it that it was the Second,because it was Early stage Great Crusade in "Fulgrim:The Palatine Phoenix"I saw the extract from Wolfbane in translation to Russian,but it's a proven source-those guys only translate books there.I would add it here too,but the book is not yet in Epub format.