Thanks to all who responded, especially Bob Hanson, who sent a scan of a Huret catalog page with exploded diagram. It seems I'm lacking the dome shaped springs that go under the decorative caps. I found that a common set of Suntour shifters (from about 1976) included a star-shaped spring that's just the right size and makes it all work much better.
Arlington, MA USA
beandk at are-see-enn dot com