It's an Edco (Swiss) hub from the mid 1990s. The freehub body has the wider spline and was fitted with an 8spd Shimano XTR cassette.

Anyway, I've just spent 10 minutes in the workshop with a die grinder and carefully removed 0.5mm from the end of the splines on the freehub body. Now it locks the cogs up tight without the spacer and I get 2 & 1/2 turns on the lock-ring. That's as good as it will get with the current lock-ring.
Just to make sure I have ordered a Token lock-ring to see if there are more threads.

Thanks for your help