These Leybold ISO-63 intake collar rings are designed to bolt-down an ISO-63K flanged fitting to the intake of a WA-251, WAU-251, WA-501, WAU-501, WS-251, WSU-251, WS-501, and WSU-501 model Root's blower vacuum pumps. The ISO-63K flange is typically used with double claw clamps. However, these bolt down collar rings and required snap ring that fits into a groove on the outside edge of the ISO-K flange allowing the bolt down collar to fasten the ISO-K fitting to the intake for the Root's blower vacuum pump. An extra sealing gasket ring (not a standard ISO-63 centering ring) is needed to form a vacuum connection between the Root's blower vacuum pump intake and the ISO-63K fitting (snap ring and sealing gasket rings are sold separately on our Ideal Vacuum web site). These bolt-down intake collar rings are design for ISO-63K flanged fittings to be used with Leybold WA-251, WAU-251, WA-501, WAU-501, WS-251, WSU-251, WS-501, and WSU-501 Root's blower vacuum pumps and have Leybold part number E361 16 120 (replaces older part number 361 16 120).