What is this black gasket

 

See the black gasket in the attached picture. It was found inside the C-drive oil reservoir. At first glance I thought it was the gasket for the top, but obviously not. It is too large. 

I think I have seen this gasket before.

Any help would be appreciated. 

Best,

CW Bill Rouse
Amel Owners Yacht School - www.AmelOwnersYachtSchool.com
720 Winnie St
Galveston Island, TX 77550
+1(832) 380-4970

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