Wmpy,
The notch cut into or punched through the blade is referred to as a "cho". The name has been used for a long time, but the correct name is "kauri". Both are acceptable. It's the same as the situation with the name kukri. Khukuri is how the knife is referred to in Nepal. One of the problems with any name used by an English speaking collector is the final interpretation and pronunciation.
Ask any Nepali what the kauri is for and he will say, "because it's a kukri and kukris always have a kauri". They like to keep things simple and in their mind, logical.
The anthropological theory is that it represents the sexual organs of a god or deity. Or a cow's hoofprint. Or a bottle opener. A sight for when the knife is thrown (the kukri is never thrown and will not come back like a boomerang)? Lots of theories, no firm answers like many stories or the "history" of any kukri.
I realize this isn't much of an answer, but hopefully will garner some more theories.