Unusual R90S and Reasonable Price?

[stonehenge]

Not sure what to make of this:

http://www.carandclassic.co.uk/car/C677028

 

[ChasF]

Not sure which bit is R90S - it' even got bing carbs!

 

[Rob Farmer]

As well as a light flywheel bottom end. There's nothing r90 on it???

 

[Beemerboff]

Not certain why they call it a 90S, but looks nicely done by someone who knows a bit about building a proper bike, and the parts alone would be worth the asking price, never mind the cost of a professional build.
Just ask BAMW----.
Wonder if the Becks boys speak English? there may be a bit more in the motor which would add value to it too.