Welcome to the history books. NC (national coarse) and NF (national fine) are "standards adopted when we were working with British made fasteners. Two countries with shared measures and still the nuts and bolts would not fit together. Had to do with the thread profiles and clearances. So standards were introduced. Before World War Two, lots of manufactures made there own bolts and screws. After, there were fewer standardized fasteners. But as we all know...Some companies kept there manufacture and standards. Try to find a 1/4 24 screw. You can find 1/4 16 and 1/4 20 any where. Harley Davidson kept the 1/4 24 screw for a very long time. As pfarber pointed out there are standards still in place. And a big thank you for the explanation of the "class 2A and 3A bolts. Clears up some questions I have tried to answer for some time.
