Well, Hobgoblin used to have an adapted guittar for sale - it had a longish, modern neck but had retained the floating bridge. I don't think anyone in the 18th century would have done such an adaption, though. Another thing to consider is when the instrument went to Holland - maybe its Dutch owner (if the owner was indeed Dutch) preferred 5 courses.
Five courses isn't that odd, is it? Such instruments had been around for a long time by the time Gibson had set up shop, so someone could have just preferred that. There are 5-course instruments in museums, but it's quite difficult to tell if that arrangement is "original" or not, without doing some serious looking. To get on or off this list see list information at http://www.cs.dartmouth.edu/~wbc/lute-admin/index.html
