I have to admit that I remove and/or snug down these lockrings using slip-joint plumber's pliers.

These don't usually appear on hi-end collector bikes, so the "rider" bikes go on happily in life with perhaps a bit of a chrome blemish there.

I've never seen a proper tool for these, but a lockring spanner as used on a bottom bracket would need to have a very different radius to fit (and not slip) on one of these stupid rings.

If only the Japanese headset makers would have taken a cue from the French designs and used the toothed retaining rings atop a threaded/toothed headset top race.