Lew
sure this is a "real" weapon and also on his book Spring says that were used for stabbing and for slashing, but on books they are attributed to the Tsonga-Venda (border line between South Africa and Mozanbico). From what I know all the axe that I have seen attributed to the zulu are shona, venda etc. The explanation could be that when the zulu had englobed other people in their reign (but to be honest I don't know if also the Venda and the tsonga were englobed) they also had started to used the axes as weapons...
... just a guess....