Ask at Velo Orange. They used to have the best info about mixtes that I have found on the web. Some time back they changed it, and I regret that I did not store the webpage, but who could guess that they would remowe it?

I remember they said that Herse and Singer "invented" the bike to be used as a fast and stylish city bike for Paris in between the two world wars (I think it was) when fuel and cars was hard to find.