just a rubber seal with a spring inside it. you need to soak it with engine oil before installing, to prevent installing it dry.
here's a picture of the front unit, from another car, being pried out with screw driver:
And here's a rear unit from another nissan, newly installed, the brown circle is the rubber seal:
Copy and paste this addy, replace the asterisked part after www. with nissan performance mag , no spacing: