I could Photochop it if you like, but essentially hold a straight edge up against the contrasting blue truck in the background and the gray part of the fin. The black line appears to stand proud by about an inch or so. It's not enough to dirupt the flow. so [shrug].
The spoon shape I referenced would have that line be a downward curve. With the box removed and tailgate closed, the top of the tailgate wold comprise a crescent-shaped wickerbill.
I did find this at
Permalink #3759 in the aerodynamics thread.
Click to enlarge and look at the rear quarters. Very similar to your plywood fins.