I just received a reply: The city archive also has no records of Hopf, but they recommended the Naturmuseum Coburg (museum of nature of Coburg) to me.
Upon visiting the museum's website, I found that they have a marble mill, but it seems like they don't have any Hopf marbles. In the picture, that the link I've attached leads to, you can see they do have German marbles, but I don't recognise any Wirepulls.
http://naturkunde-museum-coburg.de/index.php/ausstellungen/miscellanea