Once again that book comes to the rescue. Interesting that the box is all metal, makes it a bit more of a challenge. In the serial number 21-4M-F-GS-2-1 I wonder what the 21 and the -2-1 stand for. Also ,I see in the book that the 20-cwt, GS has "impact brakes" also and the hitch setup to mount the hardware looks almost identical to mine. If you are out and about with a Camera, Hanno, how about a picture or two of the master cylinder for the archives. Maybe a serial number too.
As for the wagon master, he is just biding his time ntil he can get to the keys for something.
Sean