You have been a true help to me.
So, the number I am reading on the left door column on a steel plate, is the "Coach" Number eh? Not the "Chassis" one?
I look forward to your kind reply.
That's a body shop tag, and the "1400259" may or may not match the chassis #. Sometimes they did, sometimes they didn't: This body was made by Mercedes, so chances are better that the body number and chassis number match or are close, but in other marques vintage cars, you'll commonly see body companies' tags (Karroserie/Carozzeria, etc) Notably Porsche 356s bodied by Karmann, 911s by either Karmann or Reutter or VW Beetle cabriolets also by Karmann, and the VW Karmann Ghia, bodied by the former and styled by the latter, but a "VW".
Back then, and certainly in pre-war Europe (this being essentially a pre-war car in many respects, due to the interruption of passenger car production), bodies could be changed after the car was built, and also the chassis going down the line may have been mated to a different body at the end than what was spec'ed at the start of production.
You can likely get more help from the folks at 170220.org: http://170220.org
Good luck, hope that helps. Post more pics of the car, too... Such a tease just a body tag.