Note that holes in gun frames aren't always perfectly perpendicular to the sides of the frame, either, so knowing the safety pin is perpendicular to the safety lever and base plate may or may not get you what you are looking for. I designed my own 1911 sear honing fixture long ago because of this. It lets me angle honing across the nose by half thousandths until I get even contact with the hammer hook and sear in a particular frame. No two seem to be exactly the same.
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