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Trying to figure out a fluffy way to make this work, any advice?

They got it after the Schism. Maybe they recovered it after its crew and its accompanying force was wiped out by a virus weapon.

Or come up with some other name for the upgrade that is more suitable to Space Wolves.

 Tannhauser42 wrote:
Or come up with some other name for the upgrade that is more suitable to Space Wolves.

Pretty much this.

I doubt very much that the Dark Mechanicum would have just jumped in their ships and abandoned Mars as soon as Horus died, and even if they did they would have left countless abominations behind to wreak havoc on the loyalists.

The Dark Angel and Space Wolf fleet was close enough to Terra to be of concern to Horus, they may have missed Terra but Mars would almost certainly have still been a warzone and they would almost certainly have been committed to it.