Thank you! Now I understand that it's not that I have the wrong headboard, it's that I'm missing a bracket (or two). I had a hunch that there should be some sort of bracket like that to hold the verticals in place. I don't see much value in having two of them as there appear to be in the pics; one at the top should suffice since the slots in the forward end of the bed can control movement down there. Is this bracket unique to C version trucks with drop sides?
I may take some careful measurements and fabricate a bracket like that, or perhaps see if a local guy or surplus outfit has a couple to send me (one right and left as it looks like each bracket holds 5 verticals.
Your truck looks VERY clean!
Although you're on the right track, you (probably) still have the wrong headboard.
First, here is a clearer picture of the brackets.
Ok, there is an additional difference between an A2 headboard and an A2C headboard.
The standard, non dropside headboard has a bracket, a tab if you will, that slides down into the loop that's on each end of the troop seats. It looks like this:
But that wouldn't work on a dropside-you couldn't drop the bed side(s) without removing the headboard. And the military designed the dropside to be droppable with the seats still installed, that's why the pins on each end go into the troop seats legs.
So the correct A2C headboard has a removable pin. When you want to drop, you pull the pin(s) and the troopseat(s) can disengage from the headboard.
A2C headboard: