That’s a well thought out theory. What’s remarkable is how so many completely different theories seem plausible.
What I like best about it is it also addresses the spread of them. Since they’re only found in the northern stretches of Roman territory, could be that one smith made his apprentices do it, and then some of them carried that on with their apprentices, until eventually the practice petered out.
Or not. Like you say, so many of them are very plausible.