I have the beast mostly apart.
the cone unthreads in a normal direction (anti-C/wise), the the sprocket/outer toothed assembly ('annulus' in epicyclic jargon) comes off and the notched lock-ring unwinds with an opposite thread. The sprocket has three lugs, same inner diameter as a modern Sachs one (in NZ these hubs are used by posties because of the drum brakes), but the catch is, the old sprocket lugs are square-edged, not semi-circles like a modern Sachs, or Sturmey, etc.
So I have to ask my very helpful German contact, who has already located a new-old-stock lever for the broken shifter, if he can find an appropriate sprocket. The modern round-lugged version fits like a glove but has play in rotation, so will flog itself if used in pedalling/braking

pics to follow for those other anoraks who may be interested.