This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
The large blade of my knife being so hard to open placed me in constant risk of breaking my thumbnail. To overcome this difficulty, I ground a notch in the handle as shown in the sketch. After smoothing it up with a round file and fine sandpaper, I had just as good a job as if the knife had been made that way, and it is very easy to open it, as it can be done with the thumb and forefinger. Anyone can improve his knife in this way, but be careful not to cut the notch back of the point of the small blade. - Contributed by C. M. Mahood, Warren, Pa.
Ill: Notch in the Handle