Lovely bike. I know nothing about them. Not sure if you’ve already checked but the original post above states that the number is stamped on the rear dropout.Not in Norway anymore! I brought the tandem back with me to Poland after finding it there. It's a beautiful piece, and I'm excited to start restoring it.
Looking forward to hearing from Rob about where to check for the frame number! Always great to have a bit of guidance from someone in the know.