In his book, Chicago's Mail, postal historian Harvey Karlen locates the NW or Northwest branch of the Chicago Post Office at 751 Milwaukee Ave (using the old numbering system). This would locate it roughly at Chicago and Milwaukee, south of the southern tip of Goose Island. It opened in 1874, later became known as the Carpenter Station (possibly moved fron one part of the intersection to another), and and then Station F after 1894 to 1900. In his book Chicago's Postal Markings and Postal History, Leonard Piszkiewicz goes into great detail about the branches, stations and substation nomenclature. This was the first post office that defined the Northwest side.