Learning about various thread forms is one of the great entertainments of ancient machinery (not just motorcycles). In the early days of the automotive industry there were three great centres of industry, USA, UK, and Europe. Of which Europe had several major subsets, France Germany and Italy with activity elsewhere on top of this. None of these areas really spoke to each other. They couldn't, there was only surface post and ships, let alone all the different languages, even friendly and helpful communications took months so all played alone. All had the same problems. All had much the same knowledge base, machinery, metals and men to work with and all invariably came up with much the same answers to their problems but not quite hence the weird mixture we find today.
Everyone has known the need for standardising these things for over a hundred years but we haven't managed it yet and I doubt we ever will. Human beings can be remarkably contrary.
Working on an old motorcycle is just a pleasant way of learning all this history and ain't it fun.
PS
Just be glad you haven't had to dive in to metric stuff. That's another world of mystery all of its own.
