ISO Standard vacuum fitting in size NW16. Used on the outside to clamp 2 (2) NW16 flanges and a centering ring/oring between them to make the vacuum seal. Flanges, centering rings and O'rings are all interchangable. Consideration for chemistry may dictate materials to employ. Compatible with NW, KF, QF and ISO standards. This is a building block approach for creating and supporting vacuum systems to 10-6 Torr.