Different types of down tube shifter mounts I wish to adapt down tube friction shifters to my frame. As shown in image 1, the frame is equipped with eyelets for this purpose.

Image 1 My down tube


Image 2 shows the shifter in question (I assume this is the correct order of the rings). Note how parts D and E have a snake-eyed shape---intended, I presume, to prevent external rings from rotating as the lever itself is turned.

Image 2 My friction shifter parts


My problem is that parts D and E find no match in the frame's down tube: the eyelets are perfect cylinders, with no protruding "slot" at the tip to fit the rings' snake-eyed shape. As a consequence, movement of the shift lever inevitably and almost immediately removes any friction in the system by loosening the external bolt (part F). Slippage prevents high gears from holding.

Two questions arise.


Can anyone recommend an easy fix/tweak to the problem?
Does the barrel's cylindrical shape suggest what specific tension shifter model I should look for in case answer 1 is negative? (The frame is a French C.N.C., from the 1960s I believe.)


Thanks!



Edit

Shouldn't eyelets look thus? (Note the shape, but also how the picture below's is longer than my frame's eyelet.)
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.