Ethnographic Arms & Armour - concave backside

What is so interesting about throwing knives is trying to understand just how they were used, and often misperceptions about actual use or not. In reading through Christopher Spring's "African Arms & Armour" (London, 1993) there is some very good discussion on these (p.80).
Apparantly the 'kpinga' was issued exclusively to members of warrior companies by the king or governor, thus these weapons were considered "mara ngbanga" (=court metal). There seems to have been a code of sorts in tribal combat, and these (the warrior typically had four) were used last, after spears were thrown. They could not be thrown until the warrior had declared, by shouting that he was doing so. It mentions bending the knife under his foot (presumably adjusting aerodynamics?) before the throw.

Interestingly, these kpinga were mounted inside the warriors shield on a round metal disc termed 'imgasi' (discussed by C.R.LaCae in 1926 in "Les Azande on Niam Niam", Bibliotheque Congo XVIII, Brussels).
While these were usually thrown vertically, it is known that occasionally the horizontal throw was used to circumvent the positioned shield of the opponent at lower level.

Just thought I would add some perspective to this most interesting weapon, and how it is said to have been used.