I looked up the hub specs. Does "TrackSpec High Flange Aluminum, sealed precision bearings Flip-Flop 120mm rear (threaded fixed cog one side, freewheel capable other side)" sound like your hubs? Is there a chance that this hub is a re-branded Italian Miche hub? I ask because those hubs use a lockring that is not your standard thread. The Miche thread is slightly larger diameter so a Dura-Ace lockring wlill not thread on. (I'm guessing if you force it, you are doing your best to compress the hub shell.)

I use Miche hubs. I have had SunTour 12 tooth lockrings re-threaded to the Miche standard and they work beautifully (and now represent cash outlays on par with jewelry). A good machinist can do this on a lathe. It is not a tap the machinist will have so he will be cutting from scratch and I'll guess the Dura-Ace ring is as hard as the SunTour ring if it is the thin walled bell shaped 12t cog. (Not fun to machine. It took several sharpenings to get through one lockring.)

If this is a Miche hub, the smallest cog you can run with the standard lockring is 14 teeth.

Ben