My Duster was built mid-November '73 and it IS a shock bumper. Can't tell you how much I wish it wasn't.
I have not been able to find a rhyme or reason to which cars got the shocks and which didn't. Seen it all of the place, 72bluNblu's later '74 doesn't, my early '74 does, a buddy of mine had a '74 without them, seems like I've even seen them where only one end of the car has the shocks (but I could be wrong about that).
But either way, I would bet the '73 fits a '74 if it came with brackets, but doesn't fit if it had shocks. So, I would bet he got a bumper for the shocks and needed one without, and the bumper shop is clueless that there is a difference.