It's a minefield out there http://www.classicdriver.com/en/bike/bmw-motorcycles/r100-rs/1977/115318 advertised as a genuine HPN machine.
I saved the original owners advert (I'm a sad git) when he sold it on ebay a few years ago. Built by Keith Townsend and no mention of HPN or the Paris Dakar rally.
This is the original Ebay advert:-
"First registered as a R100RS in 1977. Modifications as follows, Husky 500 upside down forks re-valved and sprung to suit bike. White power rear shocks. Swingarm lengthend by approx four inches. R80GS subframe modified and fitted together with GS seat. Stainless downpipes and modified Harris silencers (Not too loud, just right!!). Lightened flywheel and gas flowed heads.
Bad points, clutch slips under hard throttle. Cylinder heads not converted for unleaded.
I built this bike about 15 years ago and have toured, ridden it off road on many occasions and even raced it at Silverstone!!. If my memory serves me well I won the MCC Lands end class award on it twice.
It is a go anywhere do anything on it bike. Not for the novice or faint hearted. If you do not know your bikes do not mess me about with silly questions. It also comes with pannier frames and Krauser panniers.
Please feel free to view the bike, visit www.dcbutler.co.uk for our location and contact details. (The bike is currently at my shop.)"
I saved the original owners advert (I'm a sad git) when he sold it on ebay a few years ago. Built by Keith Townsend and no mention of HPN or the Paris Dakar rally.
This is the original Ebay advert:-
"First registered as a R100RS in 1977. Modifications as follows, Husky 500 upside down forks re-valved and sprung to suit bike. White power rear shocks. Swingarm lengthend by approx four inches. R80GS subframe modified and fitted together with GS seat. Stainless downpipes and modified Harris silencers (Not too loud, just right!!). Lightened flywheel and gas flowed heads.
Bad points, clutch slips under hard throttle. Cylinder heads not converted for unleaded.
I built this bike about 15 years ago and have toured, ridden it off road on many occasions and even raced it at Silverstone!!. If my memory serves me well I won the MCC Lands end class award on it twice.
It is a go anywhere do anything on it bike. Not for the novice or faint hearted. If you do not know your bikes do not mess me about with silly questions. It also comes with pannier frames and Krauser panniers.
Please feel free to view the bike, visit www.dcbutler.co.uk for our location and contact details. (The bike is currently at my shop.)"

That's the trouble, there are hundreds of "specials" out there,some more special than others, but it is so annoying when folk try it on and bluster about how "special " the one they are flogging is, when it clearly isn't. To get a genuine HPN these days would cost between £25 and 30k for a full on freshly build machine. A set of forks from a crosser and a longer swinger/shaft do not a Dakar bike make. 
