I guess after ww2, there was something of a surplus of German helmets, many of which were sold to budget-conscious non-European armies. So I guess the design became standard elsewhere.

In college years ago I helped make a short film which had a ww2 German soldier as the central character. We outfitted the actor as accurately as we could, including a genuine surplus helmet. A few of us were surprised to find that with the head held a certain way, a breeze passing over the helmet generates a fairly loud humming sound (probably not unlike the sound you get from blowing across the top of an open bottle).

It was loud enough to not only be very annoying for the wearer, but could possibly have given away a soldier's position as well, I suppose. I wonder how many German soldiers were "inconvenienced" by that little feature...