There are several reasons I don't believe this theory and I base my rudimentary knowledge on the fact I held a Supply Officer secondary MOS and went to the Installation Management course at Ft Lee, VA.
1. Small arms ammunition is a class V supply item. In this regard, it comes under strict storage regulations and transportation security. All class V items were shipped in special sealed containers and in special areas of the ship for safety.
2. It would seem to be unlikely that of all the magazine manufactures any of them would have the approval to order small arms ammunition and be able to comply with storage security on a large scale.
3. Magazines are a class II item of supply. Class II are never mixed with class V until issued.
4. The magazines would have to been shipped to a Ammunition Depot, filled with shells and re-wrapped and marked as Class V, then shipped to the port for deployment to theater. At this point, critical assets (bombs, arty shells, etc.) were given priority and small arms ammunition never was a critical asset.....so it may have sat on the dock for days.
5. Once in theater, the Material Management Command would have had the authority to have the magazines filled and issued....buy why? By the time he devoted manpower to do this job, the ammo and magazines would already be a unit level ready for issue.
6. The manpower, complexity and delays in shipment would have in my view been prohibitive.
7. Finally, when issued to the soldier, he would reload all the shells anyway as no soldier would risk his life on a little old lady in tennis shoes packing his magazines.
Just my $.02, regards, Rick.