Reading some more about the Mandara Hills I have found that many tribes relocated there due to the iron ore deposits. Either as smelters or smiths, almost a 'Solingen of Africa', although weapons were not their 'exclusive' wares.
Because of the numbers of differing tribes that inhabited the Mandara Hills, that smelted the iron and 'worked' the metal. Fortifications were built as the population increased. The smelting technology used was more advanced than many other areas of Africa. I get the impression that the smelted iron was of good quality and 'in demand' from neighbouring tribes whom would either trade for it ......or 'raid' for it. Also, as mentioned before, Mandara was often 'plundered' for slaves.
A 'slideshow' of some excavated fortifications, use the 'top' arrow to see the next pic.....and so on.
http://www.mandaras.info/StrongholdsNCameroon/fv1.htm
Regards David
As a footnote ... AFAIK weapon forms in Africa have been said to have 'spread' via trade routes and conflict. It is quite likely that in an area such as Mandara the 'intermingling' of tribal smiths also created an 'intermingling' of ideas and weapon forms, as well.