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This U-Groove holds a Hollow Threaded Rod secured by 2 nuts that sandwich this metal projection.
The Light Scocket screws onto this threaded rod and the electric chord goes thru the hollow part of
the rod & plugs into the wall..
Probably, the main reason these get confused with actual doorstops
is that usually the Threaded Rod, the Bulb & the Electric Chord are missing
and the only distinguishing feature is that U-Groove protuding from the backside