This is a government-supplied item.

【size】
Approx. 9.8cm x approx. 7.5cm x approx. 0.2cm

It was distributed to soldiers heading to 
the battlefield to pray for their long and 
successful military fortune. It says in 
Japanese that if you put it in your breast pocket, 
you can use it as a bulletproof shield,
 and it can also be used as a mirror.