This fascinating page from an unknown magazine was kindly sent to me by John Napolitano of Pittsburgh. My guess is that the DNB/Danube brand was taken over by Fuji Motors Corporation, which then mutated into Xenoah. Xenoah now seems to be called Zenoah and is part of the Husqvarna group.
I note the large number of lines on the globe that link Japan with South America. DNB derailleurs did seem to be sold in large numbers in South America, and regularly turn up on auction sites there - lending credence to my theory that DNB and Danube are linked.
I am not sure of the true meaning of “a smooth and rapid change is available because of the shifting arrangement with constant clearance between chain guide and gear tooth addendum” - but it certainly sounds interesting.