What item do the eight annex members receive as an extra for Christmas?

That item would be "butter".

The Annex was delighted to hear that we'll all be receiving an extra quarter pound of butter for Christmas. According to the newspaper, everyone is entitled to half a pound, but they mean those lucky souls who get their ration books from the government, not Jews in hiding like us who can only afford to buy four rather than eight ration books on the black market.