Dutch - I had the identical round in my own collection. I thought you actually got it from me, but now don’t recall. It is a very weak firing pin strike, and I would not stand in front of the muzzle of a gun that it was tried again in.
As far as I know, it is one of a series leading to the Panzerpatrone.
Had it not been the same lot and date as the one in the Windisch book, I would have thought it might be a fake. I had another similar type, in fact more similar to the regular eej Panzerpatronen that this one is, headstamp only, as I recall, “P M.” I assumed that to be “Polte Magdeburg.” Note that in neither case was there a dot on the headstamp. Those are just sentence-ending periods in my typed text here, not part of the headstamp.
I have no other information on either one, except to tell you that I paid a lot for the P25-code round, but not much at all for the P M round, as I recall, or maybe it was the reverse.