I can remember rear brake shoes that looked like these on lower end
bikes with caliper brakes well into the 1960's.
http://www.yellowjersey.org/
I'm not sure if the extensions added stability or just stopped the shoes from going between the seat stays and jamming.
It should be noted that besides flexible calipers, many bikes had brake bridges without reinforced holes and the holes would often tend to elongate over time.
Marcus Coles
London, Ontario, Canada.