Nice one, Graeme

...so when the mention the 'missing paperclips' in regard to secret aircraft projects, they're actually talkng about 'staples', eh?!
..But seriously, folks...
Re the Horten IV and VI: (From ’the world’s vintage sailplanes 1908-45’ by Martin Simons, 1980)
...Two Horten 6s were built. In March 1945, with the American armies 40 km away, one was test flown against the Horten 4. It was established that the performance in straight flight was as much better than the ’4 as the ’4 was better than the orthodox types. The work had to stop as the americans soon over-ran Goettingen. One of the new aircraft was destroyed on the ground by the conquerors, as were most German sailplanes at this time. The other Horten 6 and parts of several other flying wings were taken to the USA to be studied at the Northrop factory. Two Horten 4s survived the war in flyable condition. One was flown by the British Air Force of Occupation gliding clubs until it was badly broken in a landing accident. The other was taken to England, where it was flown at Farnborough but badly damaged. After repairs it was sold to Hollis Button in the USA.
Although capable of good flights in expert hands, the Horten 4 seemed to fall short of expectations. In 1959 the sailplane was the subject of a thorough study by the Aerophysics Department of Mississippi State University under August Raspet. The results were reported to the OSTIV congress in 1960. The researchers found that the flying wing had a number of serious defects. The Hortens, when they heard of these results, were convinced that the controls of the aircraft tested had been incorrectly rigged. If so, this would account for the poor results Raspets team obtained.
The Horten 4 in the USA was still extant in 1980 (time of writing). The aircraft including the Horten 6, sent to Northrops, were re-discovered in crated condition in the Smithsonian Institution’s store in 1977, and will probably be preserved.
Horten 4, 1941: Span, 20.00 m. Wing area, 19.1 sq m. Aspect ratio, 21. Flying weight , 350 kg. Wing loading, 18.3 kg/ sq m.
Horten 6, 1945: Span, 24.20 m. Wing area, 17.75 sq m. Aspect ratio, 33. flying weight, 425 kg. Wing loading, 23.9 kg/ sq m.
(Photo caption mentions that ’so far as is known, the H6 never flew in the USA’)
Existing Horten-Aircraft (From ’Flug Revue’, September 1999)
Type / Werknr. / Location
H II 2 Muzeum Lotnictwa Polskiego, Krakau (components)
H II L 6 Deutsches Technikmuseum, Berlin
H III f 32 National Air & Space Museum, Washington D.C.
H III h 31 Deutsches Technikmuseum, Berlin
H IV 26 Deutsches Museum, Flugwerft Schleissheim
H IV a 25 Planes of Fame Museum, Chino, USA
H VI V2 34 National Air & Space Museum, Washington D.C.
H IX V3 40 National Air & Space Museum, Silver Hill, USA