The Joe DiMaggio baseball card. Note the side says "No. 1"
The Joe DiMaggio Rookie Card is an old baseball card, and presumably very valuable. When all the workers at the Springfield Nuclear Power Plant get given a $5 voucher as a Christmas gift from Mr. Burns, Mr. Burns gives Homer the old rookie card (Because he said how much he hates Burns he din't know Burns was behind him). Homer takes it to Android's Dungeon & Baseball Card Shop to get money for Christmas, when suddenly the Comic Book Guy offers Homer all the money in his cashier in exchange for the card.
