I can see the spring, if it is materially different in design having a NATO number but I have a hard time believing NATO cancelled the NATO inventory number for the pistol causing CZ USA to have to stop putting it on the gun. My nonsense meter is way off the chart. CZ UB doesn't put the number on the guns, only CZ USA does. I assume to convince American handgunners the number actually means something in terms of how the gun is made, which it doesn't. The Beretta pistol the US Army uses has the inventory number so does a hammer.
The Czech Republic is part of NATO now and they use the gun, hence it gets a NATO inventory number.
The next thing we will hear is their is something special about CZ barrels that don't come with a black exterior finish.
Take Care
Bob