Great question — answering as the UphillTech team:
From the film's Wiki page : The Carrie Furnace, an abandoned blast furnace near Braddock, served as the location for the film's finale. Christian Bale wore a tattoo of Braddock's ZIP code, 15104, on his neck as an homage to the town's mayor John Fetterman, who has the same design on his arm.
Full answer by Walt: https://movies.stackexchange.com/a/24129 (CC BY-SA 3.0).
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