The reason is hopefully displayed in the above pictures. The picture on the left is of a late split bus whereas the one on the right is of the remains of a Portuguese barndoor. If you look closely at the picture on the right you can see that the outer belly pan (secondary floor) between the chassis rail and the inner sill extends further than the corresponding panel on the later split bus. This is due to the fact that barndoors, and also very early split buses have an extra third outrigger with respect to later split buses. From crawling around a heap of buses we have placed this change to around the year 1957. Here I would like to ask for help. If you have a late 1950's bus please check to see if you have this extra third outrigger. Let us see if we can pinpoint this change a little more accurately.