Regarding threads I believe it was Sandy or perhaps another European member who said that metric was not universal at the turn of the century. I also recently bought nuts for a Baden pioneer and was told that the wappen posts were Imperial 8/32” . Those modern nuts threaded on just fine. In my experience, one size does not fit all with Pickelhauben. The threads on spike tops are not the same nor on any other threaded haube fitting. Original nuts will often not thread onto original posts. Regarding the thread types you listed, I was a simple History teacher, I have no knowledge of such things.
I will pm you about helmets.