EOD - the same could be said for the standard 7.62 x 25 case, with the shoulder beginning at only a tiny fraction above 19 mm, the length of a 9 mm Para case. I believe the rounded end of the case of the cartridge shown is simply the normal way to crimp a lead bullet for many reloaders, although rimless cases in my view should always use a taper crimp, so as not to create excessive headspace by shortening the case. Further, do you know of a Polish CWS-case blank from Arsenal 21, with a “75” headstamp. They may exist, but I have only seen “77” and “79” dates on steel-case Police Makarov blanks.
I personally think, based on 58 years of reloading, that making such a cartridge out of the 7.62 x 25 mm Tokarev case, which should be found in far more abundance to anyone in a position to get either caliber of fired cases, than 9 x 18 mm fired blanks, or even live ones for that matter.
Just my opinion. It would likely take direct comparisons and very precise measurements, to come up with a positive conclusion, and may not even then, as to which case might have been used.
PS - I forgot to ask if by “blank case” you meant a piece from a draw set, or a case from a blank cartridge (platzpatrone)? I also forgot to mention that if a fired 9 x 18 mm for a noise-producing blank cartridge was used, even if sized inside and out, should show evidence of the high number of rosebud crimps (lobes) that close the Polish 9 x 18 mm Blank cartridges. There does not appear to be a trace of them on that lead-bullet reload.