Wisconsin family discovers fully-stocked fallout shelter in their back yard 50 years after it was installed at the height of the Cold War.
The boxes, old military ammunition crates, contained markings that suggested there might be explosives inside, so the family called the local branch of the Bureau of Alcohol, Tobacco and Firearms. Agents opened the crates to find... Hawaiian Punch.
Here's what the cans likely looked like when purchased as surplus:
This chest was labeled “11 AP PROJ MK 29 MOD 2, EMPTY, CU FT 1, 50 LBS”. It measures 15-1/4 X 12 X 14-1/2, has 4 clamps (one on each side), two handles and a rubber gasket. Boxes of this type were also used to ship fuses. Boxes of similar design but of different sizes were used for other types of ammunition. This is, I believe, a can for naval 20mm cannon ammunition.
And here is a reference guide to other types of ammunition cans. (Click on links labelled "Ammo Box - World War 2" and "Ammo Box - Post World War 2" for more.)