DEANB:
I wonder if after Groenenboom sold it another company had it before Slappendel bought it.I personally dont think that is a S on the door looks much more like a D but am being told that
possibly Uden operated it. Therefore this pic was possibly between Groenenboom and Slappendel ■■?
The S is placed in a white disc on a brown square with a line running horizontally through it. The wind-foil on the Marathon grille is masking the place where the letter is. Look at the picture I emailed you of the complete unit and you’ll make sense of it 
. By the way Van Uden ran trailers and you may remember the picture of Barend Sjouw’s NGC pulling a Van Uden tilt in Rotterdam docks. It is highly likely that Steef Slappendel, as a fellow O/D did the same so your info is probably correct. I believe he only did one M/E run. Robert