Bob, those windows on the passenger side door (left hand drive here in the USA) are very common. They are used to check blind spots, when changing lanes. A lot of trucks have them. I'm not sure I've ever seen a truck that didn't have one. Most drivers seem to use the mirrors more then that window though. What they do use the window for, more often then not, is for stickers, or to stick a suction cup footed Garfield there.
