[@Jbcool] From the Lexicanum:

"Alpahrius wore at least four sets of Armour before the Heresy he had discarded when they showed signs to identify him[3]. To disguise Alpharius Omegon further members of the Alpha Legion would drink a substance mixed with the Primarch's blood that seems to temporarily turn them into him. The disguise is so convincing that even the Legion's own Apothecaries are fooled by it, and those who undertake the process are even given some of their Primarch's memories.[5]"

There might be more to it than that, I dunno, just did a quick search because I had nothing better to do.