I think the current seller is going strictly by what previous sellers have stated, and is not the party who began this fantasy.
The paperwork definitely looks like made up stuff. It purportedly comes from Mainz Army Depot in Germany, with European style handwriting for the numbers 1 and 7, and references a TM9-1440-47-24P which seems to be a made up number, or at least no reference to such a manual seems to exist anywhere today. How this correlates to a Lone Star Army Ammunition Plant label on the bottom of the form is a mystery to me, but the original seller was located in Texas, so maybe that was supposed to add plausibility to their original story.
The cases themselves seem to be 37mm flare/signal cases which have a distinctive bevel on the forward side of the rim. These usually had lot markings just ink stamped on the head, not stamped headstamps, so a bit of paint remover leaves a surface ready for imaginative markings.
It is totally unbelievable that a lot of only 300 rounds would include cases made by Winchester, Remington and Lake City, instead of a single contractor, and all with similar hand stampings.
The best explanation for these is the comment by P.T. Barnum, the famous American circus promoter: “There is a sucker born every minute.”