Thanks. That's what I was kind of thinking - that something was preventing the adjustment screw from applying friction. I'll work on that. It seems to me that I recall when I installed those DiaCompe ENE shifters (From Velo Orange) a couple of years ago, I had to put a spacer (i.e. washer) between the frame and part H to push everything out so the adjustment screw could apply enough pressure. It was the same frame, so that might be exactly it, maybe a little idiosyncrasy of this frame's bosses.

In the mean time, on another forum, someone suggested that I had the domed washers (C) aligned improperly. I have them nested so the convex sides both face outward. It was suggested that they should be aligned to create a spring. The person suggested concave-concave, but that made me wonder if it should be that or convex-convex