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.
I haven't seen too many manufacturers include these tabs on a double duplex cover, but I think you will find they mount behind the recpt to grab the screw and sandwich it to the cover. The screw goes through the cover, then through the recpt, and treads into the tab. The offset dog on the tab should secure under the cover edge.