That’s an odd gap in the iron handrail of a mid-1800s rowhouse. Something’s missing. Here’s the next house over:
If you know what it is, the second picture makes sense. If you don’t, it’s not clear how that extra piece of iron at the bottom of the gap makes a difference. It’s a boot-scraper. Just before you entered the house, you could scrape “mud” off the soles of your shoes or boots. I say “mud” rather than mud, despite the barely paved nature of New York’s streets at the time the house was built, because in addition to mud the streets were full of the droppings of countless horses and mules, and, as late as the 1850s, the pigs that roamed the streets. So Brendan Behan may have right when he said that New York is “a place where you’re least likely to be bitten by a wild goat” but he said nothing about pigs.