All of these new containers had a special steel casting built into each of their eight corners. The casting contained an oblong hole designed to accommodate the most critical invention of all, the twist lock. This device, with one conical section pointing down and another up, could be inserted into the corner castings of containers as they were stacked. When one was lowered upon the other, a longshoreman could quickly turn the handle and lock the two boxes tightly together. By pulling the handle the other way, a worker could release the two boxes in seconds when it was time to discharge the
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