I’m so sorry if this has been asked a thousand times already but I‘ve searched throughout this site, Sheldon Brown’s site and also Shimano’s site and I can’t find any info about this particular headset. Or very possibly I don’t know exactly what I’m looking for?

This is a threaded headset on a 1980 Lotus Eclair and I don’t believe it’s ever been serviced. When I turn the fork it feels not just gritty but also lumpy.

The parktools tutorial on removing threaded headsets says to hold the threaded race with a spanner and loosen the top locknut but this headset doesn’t appear to have a threaded race? It has I think a keyed washer? Basically a spacer with three square holes. How do I remove the top lock nut?

Any help would be greatly appreciated!