I thought the reason had to do with the width of the bumper - i.e. they were mounted horizontally on 5" bumpers and vertically on 6' bumpers.
From Rod Diery's website:
http://users.bigpond.net.au/blitz_trucks/MODELS.html
"BUMPERS…Ford F8 and Chevrolet C8, C8A and C8AX all were fitted with 5" wide front bumpers. All other CMPs were fitted with 6" wide bumpers. Chevrolet C15s have the slot for the crank handle cut into the bumper itself. I'm not sure if the C8 is the same but I suspect it would be. All other CMPs have the crank handle guide on a small post bolted to the top of the bumper. The holes in the front face of all bumpers near the ends are for the forward facing green reflectors which were mounted behind the bumper. These reflectors can be found on Nos. 11, 12 and early N0. 13 cab trucks."
Brian