Liviu - while the quote from the website contains some good information, and I am glad it is posted here, it does not in anyway verify that Walconie Metali Sp. AKC. located in the town of Dziedzice, was represented by the letter “D” on cartridge headstamps. You say it “must have been” but no documentation is provided. Further, I cannot see any reason why, if they had a normal commerce in case-metal supply going with the Swiss firm at Dornach, why they would necessarily shut that off in EARLY 1939. The invasion of Poland in September 1939 was based on incidents falsified by the Germans to give an excuse for the surprise invasion of Poland. Of course, everyone at the time with any brains knew it was going to happen, but no one was sure of the timing. Earlier in 1939, again, I see no reason why Poland would not continue commerce with regular sources, especially the Neutral Switzerland, who continued to supply war materiel, albeit primarily to the Germans, to belligerents during the war. It is obvious that the firm at Dziedzice could not, in 1939, supply all the needs of the Polish ammunition industry in early 1939, as virtually all 1939 7.9 x 57mm Mauser cartridges produced in that year at Panstwowa Fabryka Amunicji, Skarzysko, show Norblin as the case metal supplier on the headstamp.
Well, it is an interesting and informative argument; one that is not going to be solved here. The Polish book on their ammunition still remains the best documentation on the subject that we have. Hopefully, someday, original Polish documents confirming or denying their use of various letters and symbols on headstamps will be found and end the argument, either way.