You may well be correct on that. I did reverse the order of attachment of over-labeled boxes, the Spanish label being under the German-language label, in my comments above.
It is, though, also possible that both were made simultaneously, and then as you mention, unsold ammunition from the lots labeled in Spanish were over-labeled to be sold elsewhere. I base this on the belief, from close examination, that my Gustloff-Werke labeled box does not appear to have, or ever having, a label underneath it. What is certain is that the ammo sold in the USA years later as MILSURP was primarily, possibly all, of the Spanish Label. The Gustloff label in German is much scarcer.
I have no disagreement that the ammunition could have been produced before 1938. The only positive year-date spread I could give was the 1938-1945 dates, since it is positive that it could not be made later than 1945, or in the case of the German-language label, be earlier than 1938. It would be helpful to know who the Spanish Label was made for, as that might provide an clue or even an answer, but perhaps you know and it will be in your article. The Green Chaco War and the Spanish Civil War come to mind as possible destinations and date spreads. I don’t know of any particular use of 9 mm Browning Long pistols anywhere in South America other than Colombia, which received about 1,000 Husqvarna-made versions of the FN Model 1903 Pistol, or El Salvador and Paraguay, who had some, probably a lesser number than Colombia, of 1903 pistols, likely made, as I recall, at FN-Belgium. The Colombia pistols date from 1937, so if it was for them, we are only talking one year earlier than the date-spread mentioned, if that. There is also a Gustloff-Werke, Hirtenberg box of 25-round capacity with virtually the identical wording, other than the “25” in place of “50” to denote quantity contained therein. That box is unknown with labels in any language other than German, but still could, of course, be packaging from a “repack project.” In the Spanish Civil War, as we know, just about every small arm from up to that time in the 20th Century found their way to Spain in lesser or greater quantities.
Thanks for the clarification. I was trying to offer only the minimum information, awaiting your article.