Your question on why a braze on for a cable on the chainstay if there is no derailleur hanger can be answered in two ways. One, some bikes used internal gearing like the Sturmey Archer 3 or more speed. No need for a hanger there but certainly you want something to guide the cable. See this set of photos.
http://www.hetchins.org/
Then you have some bikes that were made for external gearing but used a hanger/derailleur combination where the hanger was part of the derailleur and removable if needed. I had several of these over the years. In fact I think I may still have a Campy Valentino or Gran Tourisimo with the hanger included.
You can see that arrangement in this set of photos photo.
http://www.hetchins.org/
Ray Homiski
Elizabeth, NJ USA