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Any copy from the mid 1960s on up should be just fine. Mine has some information on the slide rule so I am sure that the newer versions have removed this section. The included angle is the angle included between the sides which is 60 degrees. The flat you are refering to is called a truncation. This is only on the top of the thread not the valley so do not grind a flat on your cutting tool. The truncation provides clearance between the external thread and internal thread (bolt and nut) to allow a tighter fit without jamming.