Solar, thanks for the explanation. I gather that all lighting & appliance panelboards have "4040" (or other) as part of their catalog numbers, so the problem was obvious to everyone but me [Linked Image]

Thanks to your pointer about the "CTL" class designation, I found a page that explained the history nicely:
http://www.mikeholt.com/misc.php?id=unformatted/8-29-99&type=u&title=Circuit+Breakers+-+CTL+Assemblies+(8/29/99)

So, I understand now that this is probably a case of using CBs marked "Replacement Use Only - Not for CTL assemblies" in a CTL panelboard.

This is a perfect example of why I lurk here -- I learn something nearly every day.

(Note to "Hitchikers Guide to the Galaxy" fans: Now we know what the ultimate question is!)