I think that is the wrapping paper for the explosive. Is the date at the bottom 19.3?

 

It was for the cylindrical explosive that went inside it, the center of the cylinder was hollow to allow the fuze (detonator) to go down inside of it.

Its very rare to find something like this still somewhat intact.

 

I pulled out my copy of IJGR&L and the Type 99 HG drawing on page 99 notes a heavy paper wrapper. In addition, TM 9-1985-4 notes the filler was wrapped in heavy paper.

 

That's a real nice bonus found in the Kiska. The markings reads Yellow (黄色) powder meaning picric acid based explosive, and the rubber stamp shows the date June 1943 and the flaming pot logo of the Itabashi powder plant.

 

Plus the 火 final inspection mark of Tōkyō 2nd Army Arsenal to the right of the date!