Jon - I don’t think it necessarily follows that there would be a known label if this was an issue round. I small issue might have eluded us all these many years. I agree with your thought behind it though - with all the digging be done by our European friends, it is LIKELY that a label and specimens would have been found if it was more than experimental or a very small issue.
Regarding the first part of my answer here, I will recall a personal incident. A customer of mine, learning I liked cartridges and 9mm very much, told me he had a couple of boxes of German 9mm with primers “painted all yellow” and a case mouth painted a transparent yellow. We all have seen cartridges where right at the mouth of the cartridge, the case lacquer is thicker and takes on a yellowish, transluscent appearance, and many of the rounds had yellow-brass primer cups… I told him, basically, that no such German type existed, but I would like to see them. He brought them in. The first thing I noticed was the 16-round box labels were buff with gree print, not the normal two shades of blue. Then I read the label and saw that it said “fur Stahlhelmabhanme.” I opened a box and was staring at 16 primers lacquered yellow. the case mouth was yellow also.
Well, not exactly general issue ammunition, but still issued in a quantity, probably for German Helmet trials that actually had no real result for the Wermacht but did design the helmet that would be later adopted by the NVA in the DDR, that they had a printed label for it. Until that time, I had never heard of these rounds, and no one I knew then in the US or Europe had either. They are still the only two box labels I know of, and every round of this ammo in collections I have traced back to those I gave my trading buddies at that time.
Long story made short - there are probably WWII labels from many countries that even today, we have never seen.
In reality, though, I agree that this ammo was not likely ever issued in quantity. Just a gut feeling that someday may be proved wrong.