eBay and Craigslist finds

This. A fixed wheel hub adapter that you screw onto a road hub and is double threaded for a cog and lockring. So you lock the lockring to an adapter that isn't, er, 'locked' itself? Am I missing something or is this as stupid as it seems?

If you were bothered about that you would just get a new hub or wheel. This product is just a compromise for people who want to use some spedicic old wheel, perhaps a disc or trispoke for track without sourcing rare replacement hub that doesnt exist or redishing it (impossible).

The adapter also has a larger contact area than a cog on its own, plus it can be superglued in place permanently.