Leybold ISO-100K O-Ring, for WS WSU WA WAU 1000, 1001 Roots Blowers.
Leybold Part Number: E23970511.
Leybold O-Ring,104.14 x 5.33 mm KFM (Viton), used for Leybold pump models WS WSU WA WAU 1000 and 1001 Roots blowers. Please see the second image for O-ring location.