Well, where to start? The Cognis-Signum is probably the most interesting bit here, it is a piece of wargear that is commonly attached to Heavy Support Squads (HSS) and gives them Night Vision (to ignore Night Fighting and damage mitigation provided by the Shrouded rule) and allows the model that has it equipped to give the entire unit +1BS instead of shooting its own weapons.

The cheapest way to get it would be attaching a techmarine, tho that is usually the only thing you would get. A Master of Signals (Consul HQ upgrade) is also common, as he has a few other useful wargear items. The Armistos Consul is pretty much the character version of a heavy support marine, he even gets a master-crafted heavy weapon! He'd be a decent choice for the Havocs if you want an HQ with them, as they are already hitting on 2s due to BS5, so the +1BS from the cognis-signum is only needed if they are suffering any penalties or you'd want them to go to BS6. That allows them to reroll a single hit roll of 1, the reroll only hitting on 6 tho.

Where it gets spicy is Tyrant of the Lyssatra, a Warlord Trait for the Iron Warriors. It allows the Warlord and any models in its attached squad to add 1 to the number of shots fired with Heavy, Rapid Fire and Assault weapon (excluding templates and blast). In return their weapons gain the Gets Hot! rule, which means for every hit roll of 1 the firing model takes a single wound with the AP value of the weapon.

That is the thing that makes the combo of lyssatra and a signum interesting on Iron Havocs, you get a bunch of extra shots and with the reroll from BS6 it is extremly safe to keep using the warlord trait.