I can't be of much help to you other than yes I think it's a badged National. If I'm wrong someone will quickly correct me. My thoughts are heading in this direction--
That is a very nice looking machine and cabinet. Why would you want to muck it up and put a dime-a-dozen machine in it? While the Spiegel is most likely a common machine not a rare valuable one, I'd choose this over a Singer 66 any day.
Cari