I was reading TM-10-35 and on pg 39 'Defense against air attack' I cam across this passage:
"Machine guns, usually available in the ratio of one to every four
vehicles, are spaced throughout the convoy so that attacking
planes are subject to a field of fire the entire length of the
column."
This actually proves my point about the MG mounts on CCKWs. 1. They were not installed at the factory, on the production line, and two, about 20-25% of trucks at the Quartermaster Troop Transport Company level would have had them (these are the QM companies assigned to Regimental/Battallion level unit). T/O&E 10-57 has the exact QM Co loadouts.