九九式特種弾頭發火 歩? (?) 一 (?)
Model 99 Special Warhead Ignition ( ? )
The Mitsubishi Ki-51 (= Type 99) was a bomber. Since the smallest bombs it took were 200kg, if the box isn't huge it wouldn't hold a complete bomb. Those would require ignition components.
When "ignition" is there, maybe/possibly cannon shells (on aircraft)? Tho the wikipedia list there doesn't show anything with a "99" model/type name, nor with a .99 caliber/1 inch size.
Or how about this: 九九式八糎高射砲 - Wikipedia
ああ、なるほど。 Now that you say it, it seems obvious. If you have a box of "ignition devices" (which presumably always means fuses and/or detonators), you wouldn't label the box with the type of plane or cannon they are used on, but rather the type of explosive they are used on. I guess none of us thought that through, certainly not me!九九式特殊弾頭発火装置二型 refers to a fuze of bomb (a long delay fuze). See "C-1(a) Type 99 special bomb fuze" in the following wikipedia page.