Leybold ISO-63K O-Ring for WS WSU WA WAU 150, 250, 251, 500, 501 Roots Blowers.
Leybold Part Number: E23970510.
Leybold O-Ring, 72.39 x 5.33 mm KFM (Viton), used for Leybold pump models WS WSU WA WAU 150, 151, 250, 251, 500, 501 Roots Blowers. Please see the second image for O-ring location.