I see how the tab holds the wedge against the screw. However, when the wedge is fully inserted, the top of the tab springs out, onto the surface of the barrel. I did try banging it with a piece of wood, and this time it did come out. However, it did gouge into the barrel a little.
The same way the tab catches on the screw, it also catches onto the far side of the barrel when the wedge is inserted all the way.
I thought maybe it was designed to hold the wedge in, to lock it in.