As stated the Vader ESB Mounting Ring ... what is it?
This is gonna sound crazy, but it always reminds me a (resin cast of a??) ring seal used on British toilet basins.
Would explain why the Empire have S*** for brains.
(not really, the Empire are cool - just couldn't miss the opportunity for the gag) :cool
If we are to go by those two pictures, then what you found is too short in diameter.