That’s a piece of the 30th Street side facade of the Wilbraham, an 1890 apartment house facing Fifth Avenue. (You can just barely make out part of the name in the fancy scrolled letters over the door.) It’s a bearing-wall building, and the north and south walls (the 30th Street facade is the south wall) are carrying the load of the floors. So this heavily-ornamented wall is structural, as was normal for this era of construction.
What caught my eye is the second bay from the left at the 1st and 2nd floors: the round corinthian column, the lintel above it, and the lintels at its mid-height are all cast iron. This is not infill of an old opening; this is the original design. Portions of the facade were designed in cast iron and the iron was detailed to imitate the surrounding brownstone. I have no idea why and in the end I don’t really care. It looks good, it’s still performing just fine 130 years later, and not every decision in architecture and construction has to be rational to the utmost degree. Someone wanted to do this and I’m glad they did. If it were not for some barely-visible seams in the iron, I wouldn’t even be certain it was iron.
Making one construction material look like another has a long history, but New York between 1890 and 1910 had cast iron posing as stone, terra cotta posing as stone, and wrought iron posing as cast iron all at the same time. It’s both confusing and, usually, good looking.
