Hitch disassembly

I'm trying to fit a novelty number plate into the hitch assembly of my TB. I have a plastic hitch cover on which I bolted the plate. This cover isn't the simple plug-in type: it REPLACES the stock hitch socket. IT has two flexible plastic 'bumps' on the sides that just snap in and will hold it inside. Hence I need to remove the steel bit (you call it a hitch socket or what?) that is held by one big horizontal screw.
I unscrewed this but found that there is still an aluminium stick holding it, from both sides (both ends of the hole). I cannot remove these, and the hitch socket is still firmly in place.
Can anyone tell me how to disassemble the whole thing?
thx
Greg