Nuclear yield calculator

Hello,

I have a chance to buy this quite interesting device. It's a kit used to estimate yield of nuclear explosion by measuring flash-to-bang gap and angular width of its mushroom cloud. Whole set is stored in metal case. All tools, charts and nomograms are in their places. Overall, the kit is in good condition and (fortunately :D) hasn't been used before. Made in Poland.

Outside: https://www.milsurps.com/images/impo...015/04/3-1.jpg
Inside: https://www.milsurps.com/images/impo...015/04/2-1.jpg
Some of the charts and nomograms (written in Polish): https://www.milsurps.com/images/impo...015/04/1-1.jpg
Is it some rare treasure or common and cheap stuff?
Thank you in advance :)