Sources on Hupfeld and others
Dr. Birgit Heise is very active with multiple projects on piano rolls and related subject. She has set-up a very good website with details on Hupfeld and many others. She has organized wonderful meetings on various topics, some of them within the former premises of the Hupfeld factory. One of the projects is focused on orchestrion rolls as a subject, that hasn't seen much research, yet. The website with all the details can be found here.