Post O-Ring(s)
This is a quality made o-ring made fron Nitrile N70 which is a durable rubber.
Nitrile is an ideal sealing compount for post o-rings as it has excellent gas barrier properties and also has good wear resistance so it should last a long time with the disconnect slipping up and down the side of it.
Losing gas can be expensive and changing o-rings is cheap. For this reason it's always good to change this cheap o-ring out periodically.
Additional Information
These are the o-ring seals that fit on the top of your ball lock and pin lock kegs. These handy o-rings also fit garden taps too so you can stop that annoying leaky garden tap a the same time.
These are a very common o-ring type and definitely handy to have a few spares.