Yes.. is the rubber ring. When I tried to connect the power plug into the socket, this rubber ring sometime "inside the plug" and made me hard to screw it.. And, I always let the ring left behind the plug so I can make the power connection much easier.
I took the ring off on my Autohelm 2000+ and threw it away as the damn thing kept slipping off the back end of the plug anyway! I hate those plugs (far too fine a thread e.g.) and this is another example of their poor design.
I have Dri-plugs everywhere else on the boat and they're far superior. Some have been there for over 20 years too!