here are a couple of shots of the Hub cleaned up.

it reads

38 P
"3 Speed"
Torpedo
System Sachs
D.R.P.
Made in Germany

I can't disassemble it yet. On the coaster brake side (lever to stay is removed) there is a large cone which is also like half of a dog clutch, which I removed and replaced. Iside, there is a large race of ball-bearings, which unlike Sturmeys, are held in a cage and don't fall out all over the floor. Very German...

On the sprocket side, I can't get it undone yet - lack of cycle tools. It is unusual - the sprocket, well-worn, looks thick at its inner diameter and I don't think will be replaceable unless I can get parts from Germany.
It does not have a large wire clip holding the sprocket on: the outer ring with the square notch appears to retain it. It doesn't have a thread so I think the key is to undo the inner cone. If my local bike shop can undo it, I will find out...