I have (had?) a KaBar USMC I bought at the little Camp Smith, Hawaii NEX in the early 1990s.
I modified it by:
1. Drilling a hole through the top of the guard and notching the blade spine.
2. While sighting through the handguard hole, using the clip point cut off as the front sight, I aimed at a point I placed on the wall to ensure my line of sight was LEVEL (level line of sight is crucial).
3. While maintaining that level line of sight, I placed nylon string in the blade spine notch I cut and let the string hang like a plumb bob (tied a nut to the end of the string).
4. While maintaining that level line of sight, and hanging the plumb bob, I pinched the string against the side if the blade and used a sharpie to mark where the string laid on the blade. This is angle zero/level.
5. Then I drew and arch (open end up) on the blade with the zero/level line bisecting the arc, and marked 10 degree increments on the arc toward the tip of the blade and 10 degree increments on the arc toward the handguard.
6. Lastly I used a dremel tool to engrave the arc, the zero angle line, and the 10 degree hacks along the arc.
This proved to be a very practical mod for me. After having done this I could use the knife with a string and anything acting as a plumb bob as a clinometer. Mostly I used it to determine the height of trees or other obstacles when laying out helicopter LZs and such.
I've looked and looked for that knife but can't find it. I fear it may have been behind the bench seat of an old Toyota HiLux pickup I had my son sell for me in 2012.