Now Feinwerkbau are back from their holidays I have been able to send them the picture shown on this thread of the superb scope mount fitted to the 601 rifle.
They say they have never made anything like this, but they are looking to see if they know anyone who may have any information about who may have manufactured this rare mount.
The mount will fit any of the 600/601/602/603 range of FWB rifles and so it would be good for me to find one, but I guess it will have limited appeal these days and so the chances of finding one must be slim.