Not sure how it slipped my mind, but one of my characters who exists in this AU actually has a similar prosthetic arm to Ferrin's, just a lot bigger and wiht only one form. remember I mentioned a weapon called Drachen Klau? it's a cybernetic arm equipped with armor shredding claws and dust-based energy wave projectile attacks.
the guy who uses it is Grey's brother-in-law. he's in jail, though. probably won't show up.