C-3PO is seen here utilizing the outer casing of an HK Guardian Droid.
The HK Guardian Droid was a droid type notable for its armor, which offered near-complete protection from blaster weaponry. The droids' unmanageability after this discovery fueled widespread anti-droid sentiment. By the early years of the Galactic Empire, HK units were no longer actively used.
During an assault on Lonn Idd's repair station by the Fromm Gang, the protocol droid C-3PO found an old HK Guardian Droid shell in storage. He used this shell to withstand low-power blaster fire from a squad of hover guards and subsequently disable the sentry droids.