The Librarius was dissolved at Nikea, which was during the winding down of the Great Crusade and before the Heresy began. I haven't turned up any specific references to Librarians existing or not existing in the Iron Hands at that time, so I expect they were present and used during the Great Crusade, hung up their Force Weapons after Nikea, and perhaps took up the mantle again after Istvaan when the Legions started pulling out all the weapons they could get their hands on.
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Duskweaver wrote:IIRC, Ferrus Manus was on the anti-psyker side of that whole debate alongside Mortarion and Russ. So it seems unlikely his Legion would have employed Librarians.
It's worth noting that the Death Guard
had a Librarius cohort at one point but Mortarion dissolved it himself before Nikea (see Morturg's bio in Betrayal). It's true that Ferrus was one of the first Primarchs found so if the Iron Hands had had a Librarius cohort before he took command and he felt strongly about it he could have clamped down on it earlier, though.