It seems to me that artistic mailboxes come in groups. Some blocks have several; the next blocks might have none. On September 14th, David Aber spotted these two boxes next door to each other.
Let's look at the distant one (on house number 6870) first. Most of it is fairly plain, but there's a beaded butterfly near the top of the shorter post. I'll zoom in digitally so you can see it better:
Here's a view of the box on 6860, more toward the front:
A lot of mailboxes are mounted on camshafts, but this is a perfect match for a '57 Bel Air pickup truck.