Thanks to the many people who responded!

With some keywords to start with I was able to track down my quarry.

The generic name for it is "line index," according to patents for the
contraption, and it is a close relative of the Kardex that was used
for issue check-in. Both of them are "visible indexes."

This turned out to be an interesting historical quest for me, at
least, because I enjoy learning about cataloging technologies. If
you're interested in the results, you can read about it in my blog
entry:

http://quaquacat.blogspot.com/2008/04/before-online-catalogs.html

--
Cheryl Boettcher Tarsala, Ph.D.

"I teach cataloging."