Thanks. I appreciate the help. Had to ask, and I now can move forward without wondering, “what if I’d asked on Bike Forums?” Agreed, the simplest solution is use the lock ring and gear that I have. I just visited a local bicycle co-op…they have bins of old parts but I didn’t get the feeling that any manufacturer ever made a locking like the one I need. Maybe it commonly exists in some other application (automotive, lawn mowers, air conditioners, pneumatic equipment???), but agreed again that holding my breath not a good idea. And I couldn’t readily identify any kit to convert a Dura Ace 7200 6-speed hub. It’ll still be a great ride as a 2 x 6-speed!