Just an update on the 116 I thought was racing in Germany. I was wrong in saying that as I have now found the details of the car, and it's the previous model, a 300SEL 6.3.
It seems the factory made only one of ther 6.3's for non factory use, and it was ordered by a private owner living in Finland, the price being roughly twice the standard 6.3's price.
While the works models were bored out to 6.9 litres, the article I have states that if a works driver used the Private car, MB actually swaped the bored out 6.9 for the 6.3 for duration of the race, replacing the 6.3 motor after the race for private racing.