Hmmm.......
Interesting. Since the source is well known, I got my information from the book "Mercedes Benz Personenwagen 1886-1984" by Werner Oswald.
On page 342 is shown the identical car (even the registration plate is the same number IIA-60698).
My book is a First Edition, and I do not know if any further Editions may have called this an error and corrected it if it was.
Certainly, the mention of replacing the motor with the 5.4 liter version is a bit odd.
On the other hand, he has a very good reputation as being one of the best and most accurate of German motor vehicle historians and certainly knew more than I do.
Bill