Spare o-ring for the transfer port of the FX Boss.
This provides the airtight seal between the barrel and the bolt housing when firing.
You should almost never have to replace these o-rings unless you often remove the barrel.
But over the years it can be worth having these!
Tip: Lube these rings with some silicone grease before you put the barrel back in the rifle for easy fitting and extend life of the seals.