Originally Posted by Camilo

To close the thread...

I had to put a very thin spacer (the thinnest appropriate sized washer I had in my box o' washers) between G and H in my parts photo above. The square part of the frame's boss stands proud of part H by a mm or two, and that little bit prevents those two parts from touching, preventing friction. The thin spacer solved the problem and the shifter works perfectly. I might take the Dremel to the boss and grind it down a little, or maybe not..