They are not for securing planks, they are the holes for attaching "bed" to the frame. As HB says, the counterbore (not countersink) hole locks the washer from spinning, the square on the bolt fits into square in washer to keep bolt from spinning.
Without the counterbore the washer would still spin. Bed wood companies can drill these for you. They are only used at the locations for tying to truck frame. You can buy a counterbore bit and do it yourself if you use the correct technique or a template.
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn
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