though I haven't found much yet to give you links, there was an auction in Paris a year or 2 ago (auctionneer Poulain Le Fur) where they sold an interesting Mercedes collection, including a 280sl 6.3 that might have been one of the MB prototype (I remember it was a white one). I tried the Poulain Le Fur site, but you have to pay to get acces to older sales results.
I unfortunately don't have much time to do the search right now, but one might be ablt to find pictures of this car.
Fabrice
Silver 1971 280 SL
New York City