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. Valve has stated they will fix this in the future.