Nyos 120 Skimmer - part identification - HELP

[elBem80]

After owning a Nyos 120 for about a year and a half I’ve completely broken it down for a deep clean. On putting it back together I found this little rubber grommet/washer in the sink. I was very careful to note the order of parts but did not see this come off and couldn’t work out where it’s from! The skimmer works fine but is noticeably louder and driving us mad. I’ve checked manufacturers diagram but drawing a blank. Can anyone tell me where this part lives on the skimmer!?

 

[Sean Clark]

That oring goes underneith the bubble plate. There is a titanium screw that holds the bubble plate in place. That oring goes under that screw.