Replacement Flat O-ring for Pfeiffer / Alcatel OME 25 S Oil Mist Eliminator, they set in between the filter Cartridge and top and bottom housings, 2 requiered.
This replacement flat O-ring sets in between the filter housings top and bottom (#6 on the diagram), they should be replaced everytime the filter media is replaced, 2 are required, see instruction manual below in Downloads.