Regarding thread differences and screws, there is a defininte difference on how certain screws fit better than others. There has also been speculation on which came first, the fully threaded screw or the partially threaded.

Have an older I-cut stock that was received with a fully threaded screw. A partially threaded will not screw in with the same ease. However, I am unsure if the fully threaded screw was "worn in" over the years or it was the original screw for this stock.

Some have also stated they may have been from different contractors and this may be the reason for the differences. Forgive me for hijacking the thread Charlie.