So this is what I understand - during the war there was a shortage of general merchandise wagons, so some 7-plank mineral wagons were converted by removing that top plank in the region above the door. That helps with unloading and makes sense to me. But then, I think independent of that, there are some mineral wagons that have about half of the top plank removed in that region. I think this is called the London plank, so presumably it is specific to private owners in that region. These wagons were still used for coal. What was the reason for doing this, and why was it regional? (I'm thinking maybe it improved line of site when shovelling out of the wagon, but if so, why just in London?)