There are several references to blood types in the story (Type A, B&O, always out for blood.) The number of vampires infesting a particular store equals the number of blood types (A, B, O, AB) that show up within the store's name. For instance, Tiffany & Co. was a 2, because it contained A and O. The leader, "a rare and versatile type that can feed on anyone", was type AB, and so he could have only been at "AB: Arthur Beren", which had the only instance of "AB" in the Square.