<a href="https://en.wikipedia.org/wiki/Encryption" class="extiw" title="w:Encryption">Encryption</a> sheet holder, KAL 61C, used by members of the military for holding AKAK encryption sheets and make using them easier.  The system was highly manual and largely became obsolete with the advent of electronic encryption measures.  This encryption sheet holder was from circa 1988.
Leonard J. DeFrancisci
CC BY-SA 3.0
<span class="int-own-work" lang="en">Own work</span>