i think its a washer to prevent any leaks
should be the o-ring to seal it.
the stock ring had grease on it .. so i just put some dow-55(o-ring grease) on the forge o-ring and was good to go.
dow-55 will make the o-ring swell thus making a better seal and extend the life of the o-ring ... used to be into paintballing and when ever i had to lube an o-ring due to inconsistancy ... dow 55 was my choice .. works great.