I see what you mean now.
Looking again at the picture of the one you're modellingon page 1, it's just possible to see that it does have this panel but only because of the weathering at the bottom edge shows it up. I have a picture of a very dilapidated van and it first appears there isn't this panel (or pressing) but it's just about evident on one side, so maybe it's down to the light in some photos.
In the previously mentioned photo, which unfortunately I don't seem able to upload in a reply, the mirrors are mounted on the front of the screen pillars but on subsequent scrutiny they're clip on retro fit ones, hence the edit.
Best of luck with the research Mark, I used to model railway stuff and that's also a minefield. Great model by the way, I'm following it with much interest.