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Those square holes were most likely cut with a broach. Same way keyways, hexagonal holes and many other odd shapes are cut. Basically if you want to make a square hole 1"x1", you drill a 1" round hole, and insert the broach( a long bar of tool steel with one end 1" in diameter and many teeth each on slightly more square than the next until you get to the last tooth which is 1" square) and ram it through with hydraulic pressure.
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