Ian H
Ancient randonneur
Ah. Proper fixed hubs have an outer, slightly smaller diameter reverse-threaded section for the lock-ring. If yours isn't one of those then that lock-ring must be off an old-fashioned bottom bracket which would screw the same way — right-hand — as the sprocket. A proper lock-ring would be about 33mm inside diameter excluding the thread. If it's not then expect to measure about 35mm.
Edit: it does look like a reverse thread, clockwise to unscrew.
Edit: it does look like a reverse thread, clockwise to unscrew.