Speaking of Cleggy's two things come to mind. One is the "painted" view from the door changes position sometimes and two the "mobile"letterbox! :
The "mobile" letterbox. How much can this mobility be attributed to the desire for dramatic effect? I can imagine Roy Clarke and Alan JW Bell plotting. "Wouldn't it be funny if we had Howard doing this. Wouldn't it be funny if we had Howard doing that." And for each variation a different position of the letterbox would be needed.
And why is it a "letterbox" when in fact it is a slot in the door with generally a flap on? Presume our US cousins know what we mean by a letterbox because they have genuine mailboxes which are boxes.