The Steel City RS-6 4" face plate has metal tabs that break-off easily. What is their purpose?
(click for full-size)
As you can see, they are cut and bent to leave a gap/slot.
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They are actually nuts, even though they are an odd shape.
They mount behind the receptacle to grab the screw and sandwich it to the cover. The screw goes through the cover, then through the receptacle, and threads into the tab. The offset dog on the tab should secure under the cover edge. This lets you use the screws on the top and bottom of the receptacle to secure it to the cover.
See the instruction sheet from the manufacturer: http://www-public.tnb.com/shared/inst/TA01413-TB2.pdf