The brass letterbox in the lobby of the Koppers Building (now called Koppers Tower, because every Building became a Tower while you weren’t looking) is a stylized model of the Koppers Building itself.
This is quite a stunning view for out-of-towners; Pittsburghers probably don’t realize how unusual it is to be confronted with such a well-preserved late-Victorian commercial streetscape, because we have quite a few of those.
This 1890 building was designed by Frederick Osterling, who also gave us the Arrott Building and the Union Trust Building. It now functions as a kind of parasite on the skyscraper Bell Telephone Building next door, but it is still an impressive work of architecture.