They don't PLUG into the turbo, they thread into the turbo and are called banjo fittings. The fittings (bolts) themselves are hollow through the threaded section and two holes at the top. They fit into the banjo section, use a 17mm head and a M12 1.25 thread pitch.
^ not the IHI RHB5 VJ11 fittings.
EDIT: the top is inlet, the bottom outlet.