I've heard François Cevert's name mentioned as the mystery driver, but this seems to ignore the fact that he was killed at Watkins Glen in 1973

As for the car, it was one of those beasts of an S class. My mate has an 80s 560 S class and I can confirm that it is in no way impeded by its size and weight. I think the 6.9 is even hairier. And, if my middle aged memory isn't faulty, it was a Mercedes-Benz 450SEL 6.9 that did some sterling work in the seminal French car chase film 'Ronin'.
GC