Sorry I haven't really responded to this thread in a long time. The fact that I couldn't find any further info on Hopf, even when asking the city/state archive of Coburg has basically stalled my research completely. I do now have Renée Holler's "Murmeln, Schusser, Klicker" book, but it doesn't reveal more about the company, except for the marbles they made. I find the marbles on the book's cover a bit hard to discern from Veiligglas. I think the ones from Hopf appear to have less crisp definition between the ribbon colours, but even that is disputable. I couldn't find any other differences.