Treshkin - the use of the letter “P” standing alone as the factory designator on 9 mm Para cartridges is incredibly complicated. It was not me that did all the great research on this group of cartridges, but rather Lewis Curtis. He sorted the outh very well, no easy task since this “P” was used by Polte, Soluthurn, Palencia, and possibly by FN on ammunition made for Cartoucherie Belge.
With the Specific headstamp “P 24” the choice narrows a little. Palencia of Spanin is eliminated, even though they used virtually the same headstamp on 9 mm Largo ammunition. The four companies involved seem to be Polte, Patronenfabrik A.-G. Solothurn of Switzerland, Cartoucherie Belge, and FN.
Regarding FN and CB, the cartridge with no serif line across the bottom of the vertical stroke of the numeral “4” is the cartridge related to them. It is also related to Switzerland. The best thing to do is to see the Curtis Book "9 mm Parabellum Case Type and Headstamp Guide, , Vlume III, Part 6, which covers among other letters, headstamps with the letter “P” as the primary entry. All I could do is simply retype it here. I will say that this headstamp, with the serif-4, is found with FN bullets in it, but there is also a headstamp “C B 24” that has the identical numerals. The “P 24” headstamp of this style has been found in boxes with a Dutch Label, “Patronen Scherpe No. 5, Luik 1923” overlabeled "328, the Swiss Identification number for 9 mm Para ball ammunition, and with a handwritten addition to the label “v. F.N. Herstal fur J.J.G.” "J.J. G. are the initials of the Director at Thun during the period 1923-1924.
The “P 24” headstamp that has a serif-4 is attributed to Solothurn. It is usually found loaded with a Swiss bullet, powder and primer. There is also a Swiss-Style box label for it, which Curtis pictures. However, it has also been found that both headstamps are found with a mixutre of primer, powder types and bullets, not typical of either Belgium or Switzerland. Further, these “Mixed loads,” as Lew calls them, have been found in Polte Military boxes together with 1926 and 1927-cated Polte rounds.
Lew’s final conclusion seems to be that both headstamp styles were primarily made for Germany in the immediate Post WWI period, and delivered to Polte to support their post-war production. The mixed loads are likely to be a product of Polte using a mixture of components from the two suppliers.
There is also an “x 24” headstamp, which is related, but that is another story I cannot pass on right now - the information on that also was sorted out by Lew.
Again, please note that NONE of the above is resulting from any research I did. This is simply a quick summary of the work of Lew Curtis.