Sorry to be offtopic.
But its not Captain Keyes.
It says it somewhere on Halopedia, That each skull represents a Dead or Missing Spartan.
I see where you are coming from, as Keyes has the "Port" on his head, as you see in a scene on Halo CE.
But the spartans all have this aswell, Like how Chief has a slot in his head, where Cortana can be implanted.
Each spartan has their own slot incase of Emergency AI Use or Important Storage.
Go on Theatre and look at the Skulls, youll see the port on the back of the head.
Back on topic. The skull looks alot better without the jaw. Almost a prefect Replica