In an incredible stroke of luck, while researching the first owner of my "slate gray" 911, Porsche Germany confirmed to me yesterday that they first send the car to their experimental department before delivering it new and tweaked to its first owner, Mr. Jo Siffert himself!
I always suspected the car came from Siffert's garage as it was first registered in Fribourg Switzerland, but I never imagined it could be his personal car! I'm now checking with the factory the precise circumstances/reasons for this factory gift to Siffert....