Mortice is happy for a chance to explain! "Our Titan's former identity is a secret - my understanding is that the Titans' identities are slightly more accessible in the other factions. In ours, only the Engineer Prime and the Titan itself know. Not even the Imperator knows. To achieve and maintain this level of secrecy, the identities of all the Titangrinder candidates had to be kept secret once the final selection was made, otherwise it would made be obvious by process of elimination.
"To do that, the candidates were given new serials and their previous serials redacted. Their shells were partially deconstructed and mixed with the other parts, before they were re-assembled. But of course there were 15 candidates, plus the Titan's former shell. No matter how you mixed the pieces up, there would be a whole shell's worth of parts left over. This is that collection of parts. It's an 'Anti' Titan in the sense of being a counterpart, not in the sense of being against it. It's a physical remnant of the Titangrinder - there's at least one piece in this assemblage from every candidate's shell, including the Titan's old one.
"We keep it for its sentimental value, I suppose. I think if we used it for a new unit, they might feel they'd had unfairly high expectations forced upon them."