4-1/2" x 4-1/2" Single Acting Spring Hinge Set
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A replacement for that hinge would be be Stanley 2060R 4 1/2" x 4 1/2" Spring Hinge as seen here:
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The finish that you have I believed to be us for which stands for satin brass.
It is very possible that that hinge is UL listed for use on fire doors. Although I do not see evidence of this listing on the face of the hinge, and you would only know that positively by doing the back side and inspecting for engraving that indicates it as such, (the reason I bring this up is because if your door is fire rated absolutely) replace it with a listed and labeled fire rated approved closing device, which the Stanley hinge is.