4" metal face plates and box are very common for exposed wiring. Receptacles connect to the face plate instead of to the box, which is different than in a house. The face plate comes with a few screws and nuts for this purpose.
It's possible to attach the recep with 2 screws and nuts at the top and bottom (red arrow) or a single screw in the middle (blue arrow). Which is the best way?
(click for full-size)
Best Answer
National Electrical Code (NEC), provides the answer in article 406.4 (C).
In your example image, the receptacle mounted using only one screw is a code violation.