The Antenna Topper is a jiggleboned item that attaches to Atlas and P-body's antenna. It was available as a bonus for preordering Portal 2 through pre-ordering through Fnac (France), Webhallen (Sweden), Alcom (Swiss) or Softridge (Swiss). Just this bonus is a supplement to Russian retail version of Buka. Blue and Black version. 
Currently, the only way to display this item in-game is to have nothing equipped in its respective loadout slot. It was stated by Valve on April 22, 2011, that they were "working to correct this and make the DLC appear as an item just like other skins, hats, etc. in your backpack." As of 2019, this correction has not been issued and the only way to display the item remains the same.