Okay, so what I thought was a ball-ring turned out to be the driver, which has no threads but keys into the brake with 2 ramps. Attempting to undo the hub by locking the brake arm and forcing that would have damaged something for sure.

This is the hub dismantled:
[IMG]
IMG_7628 by Dawes-man, on Flickr[/IMG]

I'd wondered what the square hole in the 2 Sachs spanners I have were for. And duh, had wondered why one end of the hub axle was squared. The hole is for locking the axle when you undo the knurled locknut on the brake arm side. I had to put the spanner in a vice to get the locknut undone. I've discovered that the only chrome loss on the hub is on the squared end, which suggests the hub might have been dismantled before:
[IMG]
IMG_7629 by Dawes-man, on Flickr[/IMG]

Ah, yes! The 73 year-old grease... this shows how the brake arm end driver keys into the brake drum:
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IMG_7632 by Dawes-man, on Flickr[/IMG]

And this shows how the drive-side driver keys into the brake drum via ramps - both these parts will dismantle further but I'll do that when the parts are clean and I can see exactly how they come apart and fit together:
[IMG]
IMG_7630 by Dawes-man, on Flickr[/IMG]