Assuming the rifle is not a Nylon 66 but one of the other Nylon stocked rifles (Nylon 11, Nylon 12, etc.), the rear sight attaches to the barrel. Check the hole spacing. If it is under an inch, the sight is probably the simple spring steel type with an elevator and no windage adjustment. If the spacing is wider, it is probably the sight that was put on with the "ray gun" front sight. Those are more elaborate and Gun Parts lists them at around $35.