ace73 you are being silly. The NSN number is put on by CZ USA as a marketing ploy to convince US shooters there is something special about a NATO inventory number. Shovels have NSN numbers, so do Inglis Hi-Powers used by the Cdn. Army. The Inglis pistols were made prior to NATO being formed circa 1944 and 1945 and have not been made since. The Cdn Army still has unfired Inglis pistols in stores. If there are PO-1 pistols without the number they were missed by CZ USA. You want the number stenciled on the slide send it to CZ USA I am sure for a fee they will be happy to oblige.
Every piece of equipment used by NATO member troops has such a number. When you run combined operations under NATO command the sytem allows for example a US supply officer to send out a German infantry unity the right piece of equipment.
As a buyer the last thing I need on my pistol is some useless inventory number marring the surface.
I have no idea where the explanation for the absence if a number on some pistols arose from. Probably from the same source who came up with the idea the inventory number meant something more than it does. I know, lets all say "NATO" tested the pistol before allowing the gun in inventory. Some folks in the US still believe the world is flat or just 8,000 years old so why not that story? It sure seems to have caught on.
Take Care
Bob