My experience backs up what R1 has already said. I do not believe that there was any set date in play here. It all depended upon such things as price and the manufacturer involved. I have a great private purchase pre war JR119 helme with officer liner,wappen and OR trichter. The wappen has split brads soldered to the back and not screw posts.
All of the split brad stars I have seen have been associated with mid war officer helmets. One last thing to add, is that there are manufacturers' variations in the OR's split brads as well. As soon as I have time, I will post some pics of 3 variations that I have in the collection.
Brian