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In the Legiones Astartes rules for iron hands, it states "any iron hands character can take a cyber familiar for X points"
Now a cyber familiar increases an invun save by +1 (4++ becomes 3++)
Now Ferrus Manus IS an iron hands character....
And he has a standard 3++ invun (so that means he has a 2++ with the familiar)
Can he take one?
(I might have to email FW)

I would say not as Ferrus Manus is a unique character, therefore you can't give them any other gear than what is listed for their entry.

I think it is possible due to the wording of the rule and fluffy too .

Cyber familiars only boost an invul to a maximum of 3++ so it would do nothing for him

Some unique characters have wargear options, but they're usually listed on the characters page. I don't see a rules problem with a free for all upgrade like this.
Whether or not Forge World intended that to be an option I can't say though. Not that it matters in this if Melcavuk is correct.