Rodneyb,
You meant countersink, right?
The ANSI standard is for a maximum theoretical sharp diameter of .172" and a minimum edge of .146". This differs from the metric standard in which the head height is significant, and the diameter is to the outside edge. I have a 1941 drafting book , Engineering Drawing, by Thomas E. French, which quotes a head diameter of .164" without tolerance, for something called American Standard.
Is it possible that your original drafter measured an available screw?
JHG