NavList:

When you turn the light on it lights the plotting surface from a center lightbulb. When you open the cover that light goes out and a light bar on the left side of the cover illuminates the interior of the kneeboard. The card inside with the 30 degree sectors was for use with the YE-ZB homing system which used spread sprectum technology and VHF freqencies to prevent the enemy from being able to detect the signal and, even if they did, to home on it to our aircraft carriers. The carrier sent a different morse code letter into each 30 ° sector from a rotating antenna. The pilot marked  a letter into each of these segments prior tp take off based on the code for that day so, when hearing a letter, he could determine which segment of the compass he was located in in relationship to the ship and was then able to fly towards the ship. Since the code changed every day, even if the enemy could hear the signal they could not use that information to attack our carriers. There is a similar circle on the Mk 6 plotting board.

http://timeandnavigation.si.edu/multimedia-asset/zb-1-radio-homing-adapter-and-security-cover

http://aafradio.org/docs/YE-ZB.pdf

https://airandspace.si.edu/collection-objects/chart-radio-frequency-united-states-navy-ye-zb-vhf-nav-radio-range

https://timeandnavigation.si.edu/multimedia-asset/ye-zb-chart-new-hebrides