The allusion is probably to Graham Greene's novel _Brighton Rock_. There
were contests held by British newspapers in the 1930's where they would have
someone walk around a town on a given day. His name, his picture, the day, and
the town would be given in the paper. When a newspaper reader spotted him, they
would go up to him and say, "You're [whatever the name was] and I claim my
five pounds." They would thus win the contest. This is a plot point in
_Brighton Rock_. So the phrase now means, "Hah, I answered that question, didn't I?"
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