As far as I know this concept was proposed some time in 2004/2005 and was subsequently dropped due to the poor sales of the SLR & souring of relation between Mercedes & McLaren on the road car program.
Don't know the current status but the chances of this car ever being launched were really low after departure of Dieter Zetsche and his replacement Eckhard Cortes was not really keen on such expensive niche products.
But anways will poke around some sources & see what turns up.
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