This unfinished droid is the product of Anakin's genious. Built to
assist his mother with household duties, Anakin modeled him after the
Standard Galactica protocol droid.
Although not entirely completed C-3PO can still perform several of
his programmed tasks. A few minor parts are missing, along with his
entire outer casing.
Additional pieces are added over time and he becomes more and more functional. Eventually he can go out and start fulfilling his destiny, which he was an able to do previously due to the fact that his exposed circuits couldn't have with stood the destructive force of the outside world.