I was going to call this post "Variations on a Theme" or maybe "Knock, Knock," and then it occurred to me -- hadn't I already used these titles? The answer is yes, and the hard truth is that I am repeating myself. Oh the tribulations of advancing age. So we'll just leave this one untitled, and let the index sort it out.
I spotted these two mail slots within minutes of each other in the amazingly varied 17th arrondissement. I quite like them both.