Yeah, I remember both of those. The red ring was crafted to work with the once ubiquitus Sigg fuel bottles, matching with the vent holes. It may have been made by a third party, not Sigg, I simply can't recall at this point. I probably have one in a random box but I've pretty much stopped toting WG stoves and when I do, they're pump-in-bottle types. The other was a standard Sigg plastic screwtop with a tube inserted and an added vent hole. Had one of those, too.
For extra geezer points, the older metal-cap Sigg bottles had slightly different threading and the plastic cap didn't fit too well, while the red ring didn't work at all.
I do - I actually had one, and it worked well. There was also a full spout that had a small tube coming out of it for pouring; it worked great, but didn't have a stopper, so you ended up carrying a stopper-cap and a pour cap on a loop of cord, just like the flat disc.
As I recall, the original Optimus stopper spout had a couple of holes down in the threads; their idea was that you unscrewed the cap a bit, and could pour fuel out one hole while using your finger on the other to control the flow rate. It would have worked great, except for all the fuel leaking out around the unscrewed edges.
I haven't seen one of the discs for years, however.