Reynolds made tubes in other grades, not just 531. A frame, its forks and stays may all be 531 or only the main triangle, the forks and stays being made from a lesser grade tubing. 531 may be butted or not. Forks are taper gauge with a butt at the top, stays are not butted. Your bicycle may have a plain gauge 531 frame and 531 forks etc, or a butted 531 frame. If your bicycle is made from butted 531 only the top, down and head tubes are double butted, the seat and steerer are single butted. On a large frame it is not uncommon for the seat tiube to be plain gauge, while the other trubes are butted.