These adapters are KF-40 to 2 inches NPT-male. The flange size follows ISO-KF standards and is NW-40. They are commonly used to make connections between ISO-KF fittings and 2 inch NPT-female fittings. The ISO-KF flanges are often called quick flange fittings because they are quick to assemble. A circumferential clamp and centering ring form the vacuum seal between the connecting flanges (please see top right figure - click to enlarge). The centering ring contains a rubber elastomer O-ring. The standard sizes for quick flange fittings are KF-10, KF-16, KF-25, KF-40, and KF-50 with flange sizes NW-10, NW-16, NW-25, NW-40, and NW-50, respectively. They are made of Stainless Steel.