[davemc]

Thanks to Mike's photos, I got the nerve to take the shadow meter apart.  As suspected, one half of the vane was missing - obviously broken off. Checking with a small magnet, the remaining half of the vane appeared not to be ferrous, or at least it was not attracted by the magnet.  I cut a small piece of paper thin brass and glued it in place (approximately) - not an easy task if your hands shake! I finally was able to tack it in place using fast setting epoxy.  Once dried, I painted it with flat black enamel like the rest of the parts. I did notice that the brass half of the vane was heavier than the original half.

In any case, it works perfectly now!  Thanks for all the help and advice.  I'm guessing that someone stuck something into the open front of the meter and in the process broke off the front half of the vane (the half that faces the back of the dial opening). The weight difference and resulting unbalance apparently did not matter.