The Quarryman Brownstone is the headquarters of the Quarrymen, located in Manhattan. It was here that John Castaway recruited the first Quarrymen, including Vinnie Grigori and Sarah Browne. Castaway keeps an office here as well. ("The Journey")
The front door has a Quarryman Q emblazoned on it. The building is number 443, which is written on the right of the door as you enter.