I wish I could help.I remember getting an extra O-ring with a seal kit for something which I know I never had to use, and the 180 sounds about right but I just don't remember.Maybe an extra seal was included accidentally?
Most 180's I've seen have an O-ring groove at the front of the valve body that takes the same ring as the CO2 cap, with a smaller ring sealing the assembly thread. The factory manual shows a "new valve assembly" like yours with the single ring at the joint being same as the cap ring, including an admonition to leave the assembly loose until positioned correctly in the tube, then tightening will seal it all.I'd expect your spare ring to be the smaller one for earlier valves.Don R.