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