Regarding the “dou.” code 9 mm Kurz with nickel bullet, no mystery there. For what was likely a small production item, the Czechs simply used the .9 mm Kurz bullet they had been making all along for their own CZ vz. 24 pistol which was standard in the Czech Army before the German occupation. They, and the German accepting authorities, likely had no problem with this at all. There would be no reason why the bullet “had to be” of the GM or GMCS variety. The undated “dou 7.65” (my only “dou” round that omits the period from after the code letters) 7.65 x 17 mm Browning cartridge also has a CNCS FMJ RN bullet. Typical of Sellier & Bellot!
I think you are on the right track in looking at when these rounds were made and the huge military buildup that continued into those years in the Third Reich. I agree that the use of color seals on coded-case ammunition, along with commercial Sinoxid primers, is not explained by that. Perhaps they were loaded for external use (outside of the borders of Gross Deutschland) of police. Criminal Police and other groups like the Gestopo were active beyond the borders of Germany into the occupied countries.
Since you mentioned many of the nickel-bullet 9 mms casually as an example, I am not sure what you need from your request if any of us have coded-headstamp military rounds with nickel-plated bullets. I will list mine, even though it is a bit redundant to your comments
P14A 40 12 *
P14A * 52 40
P.405 * 1 40 (Red seals and “O” primer)
ak * 21 41
ch 2 41
ch 41 3 (these “ch” rounds are the only ones with CN, rather than CNCS, bullets)
ak St* 8 45 (red mouth seal. I think we both agree these are Czech post-war loadings using left over primer cases.)
ch St+ 1 42 (like the brass-case ch rounds, non-magnetic bullet)
dou. St + 26 42
In your comments about the P.405 cartridge your statement “This is not a cupro-nickel (CN) coated steel bullet, but the bullet jacket is clearly nickel plated …” I don’t follow this. The bullet on these rounds, lot 1 of 1940, are definitely what we call in our non-technical way, CNCS. That is, they are nickel-plated over a steel jacket. That contradicts your statement that the bullets are not a cupro-nickel (CN) coated steel bullet, but the jacket is clearly nickel-plated. Isn’t “plating” the same thing as “clad” within the parameters of this simple way to describe a projectile?
Regard Sheryl’s “nickel” round, could he be he is simply speaking of the projectile, and not a completely nickeled cartridge like some Exerzierpatronen. I am anxious for his reply to find out. Like you, I have never seen a “ch”-code nickeled dummy round.