So this is the notorious 'Blue Box' which lurked in the basement of the NSU dep't at the Audi plant in Neckarsulm, containing one ultra-rare NSU Works Four-cylinder RS54 dohc 500cc engine. The engine itself is awaiting some remedial work; while the critical components are in place and in good condition (even the 2 magnetos and 4 Amal-Fischer RN carbs, now priceless - I tried to find a pair for an R51SS, but no dice). The engine isn't in the chassis for this machine as it lacks a
sump casting; I'm sure Wolfgang will correct me but I understand this particular engine was used for another purpose (or perhaps just testing), at the end of its racing life, and the sump was removed, and ultimately lost. It will be a relatively simple matter to cast a new one, as it doesn't support the bearings or oil pump, just a cover essentially, and at that point it will be reunited with the chassis. I can't wait to hear it running. 
The photo below is from my own archives; it was taken by a US soldier who was an amateur photographer / motorcycle enthusiast, and shows the NSU RS54 in action at the Nurburgring ca 1951, and is probably Heiner Fleischmann aboard. Note the enthusiastic response of the children in the background! 'Go!'
In the two boxes marked 'Spare parts for T.T.' were other NSU gems, including one of the early R11 Rennfox 125cc
engines from 1951, with shaft-and-bevel driven dohc, and the funny canted angle of the cylinder head, desaxe the crankcase; see the photos of the whole machine, taken at the Deutsches Zwierad Museum in Neckarsulm (just a few minutes away from the Audi plant, and with a wonderful collection of machines in a converted Schloss). There has been much speculation about why this cylinder head was designed off-angle in this way, but the simple truth is the pressed-steel frame wouldn't permit 
this large head to fit in the frame any other way. In later models, the cylinder head was redesigned, and sat straight relative to the crankcase.
The R11-51 model also used a petrol tank which was stretched and modified from the original steel roadster item, and lacked the graceful hammered alloy bodywork of the later models.
Still, it has a rustic charm, and was certainly effective on the track, producing 12.5hp @ 9500rpm. I'll make a more thorough investigation of the Works versions of the NSU Max/Fox models as time permits this year; their history is amazing,
including the all-conquering 'if it started the race, it won the race' Rennmax model of 1954.