I don’t know anything about this ammunition being developed for “testing MP38s and MP40s.” I have found most weapons collectors to be very ignorant on ammunition in general, unless they also collect it, and with lots of fancy stories about the history of cartridges. Most of this ammunition was contract during WWII for various countries, including the United Kingdom and China. The 64-round box in our collection is dated “30 June 1942 Lot 176.” It was designated “Ball M1” after April 2, 1942, and certainly was not made just for testing captured SMGs, if that was an intention in its development at all. This ammunition was made in far more than weapons-testing quantities and supplied for combat use to various countries. “History of Modern U.S. Military Small Arms Ammunition, Voume II: 1940-1945,” by Hackley, Woodin and Scranton, mentions nothing about this ammo being made for testing German SMGs. It does help us with dating it, as it mentions it was initially supplied to England before the entry of the U.S. into WWII, so it could have been made as early as 1939. However, I would think that since the Sten gun was developed after Dunkirk, due to the huge loss of small arms there, that 1940 is probably a more logical starting date for this headstamp and cartridge.
I have white contract boxes in 50-round English Label, 50-round Chinese label, 64 round English label, and a 50-round box for definitive proof loads, which have the W.R.A. 9M-M headstamp, but have a tinned case.
Cases with this headstamp and the same nickel primer with blue seal are also found loaded with commercial-style truncated FMJ GM and truncated FMJ-HP GM bullets, packed in commercial boxes, probably for sale during WWII. One box is what Giles and Shuey in “One Hundred Years of Winchester Cartridge Boxes, 1856-1956” call a “1932 box style” and the other is a “1939 box style.” Of course, depending on caliber the 1932 style was used later than the 1939 style, and the 1939 style was used at least until 1945, and perhaps after that, again depending on caliber.
I think, though, that we can pin this headstamp down to the WWII years - probably from about 1940 until 1945 or 1946. I am not sure of those dates, but from the box styles it is found in, I would think they are close at either end, earliest and latest dates, perhaps within a year or so either way.
I cannot do it now, but later I will have Joe post pictures of all of these boxes. The proof box has been pictured before as has, I believe, the Chinese label, but I will repeat them in favor of continuity on this thread.