There don't appear to be any appropriate chains in hardware or craft stores around here, so I made this one from two feet of 3/32-inch brass rod. Annealed and coiled around a 1/4-inch nail, it made just 20 links. One of them, detached from the end and forming a side link to attach the tab (via its own ring) to the chain, is located nearer the top scabbard ring so as to allow the scabbard to hang at an angle.
The tab, of course, is just a scrap of veg-tan leather, molded, dyed, sealed and riveted in the back.
And there you have it. I guess if I ever get zapped into a Horatio Hornblower novel and somehow land a commission instead of having to carry up buckets of bilge for a living, I'll at least have one thing taken care of.