These adapters are KF-16 to Inlet or Outlet Welch vacuum pumps, models 1399, 1400, and 1400N, Old Style Pumps. The flange size ISO-KF NW-16 to Welch pump thread size 5/8-18. They are commonly used to make connections between Welch vacuum pumps and ISO-KF Flanges. 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 corrosive resistant stainless steel.