It's also used in Bingo apparently, got this from a Bingo site about the origin of the bingo numbers slang terms:
50. Bulls Eye: This piece of bingo slang comes from the game of darts, which for those who do not know, is a popular game in the United Kingdom which entails throwing small, flighted darts at a circular board divided up into numbered segments. The centre of this board is typically a small, circular dot called the "Bull's Eye" which, if hit, is worth 50 points. Hence 50 = Bulls Eye.