C-3PO is depicted wearing the outer casing of an HK Guardian Droid.
The HK Guardian Droid was a droid type notable for its exceptionally strong armor, which provided near-immunity to blaster weaponry. Their intractability after this discovery led to increased anti-droid sentiment and their discontinuation by the early years of the Galactic Empire.
During the Fromm Gang's assault on Lonn Idd's repair station, the protocol droid C-3PO found and utilized the old shell of an HK Guardian Droid from a storage area. This allowed him to withstand the low-power blasters of a group of hover guards and neutralize the sentry droids.