Different types of down tube shifter mounts
I don't know anything about the history or whys, but I have seen shifters by huret that match up with this type of mount (as @danielrhicks was getting at):



As you can see they have a square baseplate that acts as a stop for the shifter. I had always just assumed it was some sort of French standard that didn't last very long. I have also seen simplex shifters that have a similar design. Often times I've found these frames have a different thread pitch than shifters designed for non French frames as well, making it extra difficult to mount another shifter.

obviously a second, less than ideal option would be to use a clamp on shifter set lower on the downtube.

thanks, France.