What goes on atop a Manhattan building is a mystery to all but that building’s inhabitants — and the neighbors in taller buildings looking out their windows, of course. You don’t necessarily expect a 70-year-old pre-war building like 405 West 57th Street to have such a relaxed, fun and communal retreat on its tippy top, but it’s the unexpected that makes peeking into older New York City buildings so interesting. And buyers will have their chance to peek on Sunday, when there’s a scheduled open house for a 1-bedroom apartment in this co-op building. Go ahead and tell us if that rooftop looks as pretty in person.