My friend, the Danish maker Jens Stenz has looked at this posting, and attempted to leave a reply, but apparently was unable to do so. He thinks it is possible it was made by the German/Danish maker Carl Felszner. He also says the Royal Danish Orchestra is the oldest symphony orchestra in the world, dating to before 1500. He said the orchestra maintains a listing of each musician who is/was employed there, and that it may be possible for the OP to trace their great-grandfather's name from that list. Jens is willing to help further if needed, and can be reached at jens@stenz-violins.dk