Why the little slot in PTO Guards?
That is where the guard attached to you implement hooks up.
The old style PTO shield is shown below. The shield was generally three pieces, the two shown and then a longer section that went all the way back to the implement--and that section telescoped on itself.
When you hooked up you pulled the center joint of these two pieces up, slipped the end (left in the lower picture) into the slot on the tractor PTO cover and turned the flat iron piece so it was behind the tangs on the tractor cover. You then pushed the raise joint back down to the straight position and you were ready to go.