Gerd's commercial vehicles are always a great challenge and often eat up hours of research time.
For a little "Comic Relief" on this Sunday, here are two photos I copied off of Google whilst trying to solve this dumper puzzle.
I must say I was totally amazed at the number of "enthusiast sites" on the Internet that are devoted to such subjects as "Mining Trucks", "Coal Trucks", "Dump Trucks" etc. etc. etc. And many of them have a great sense of humour in their postings as you will see below.
And, BTW, I am no closer to solving the puzzle but it has been fun learning more about these trucks.
Bill