The Primarchs were scattered through the galaxy by the Chaos Gods and then found one by one by the Emperor during the Great Crusade.
It is well-known that Horus was discovered first. This gave time to the Emperor and the future Warmaster to develop a deep bond.
We (at least me) learn at the beginning of Wolfsbane that Leman Russ was the second to be discovered.
I believed, and Wikipedia also claims it (but without a source), that Alpharius was last to be found. However, I have a slight doubt about it since everything with the Alpha Legion is "shrouded in mysteries". Also, from Lexicanum,
before Alpharius' discovery by the Emperor, members of the XXth Legion were already using the codename, as seen during the Rangdan Xenocides.
Does it mean that Alpharius had already been discovered by the Emperor but his discovery kept secret for a while?
- According to Lexicanum, Magnus the Red was with the Emperor when Lorgar was rediscovered.
What other information on the order of rediscovery of the Primarchs do we have?
I am looking for absolute (like "Leman Russ was the second to be discovered") or relative ("Magnus was discovered before Lorgar") information.