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Well so far no luck in finding out more.
I've not managed to look up F.C Zwanzig here in Denmark (Schleswig - before 1864, when we lost it to Germany)
But have been able to get confirmed from a 3.party that It's correct that there was a Friedrich Christopher Zwanzig (1822-1889) in Schleswig, as well as his son Friedrich Christoph Zwanzig (b.1857) , Thanks to Dragonflywink !
Just would had been great to actually see his name/hallmarks in a book.