Incredible forensics there David!!!!
That is amazing detail, which you always present so well. I think this would be thinking inside the box!
so it seems you are a master of that, outside the box, and circles around the box!!
I must admit I really admire you guys and these mathematical observations, while all I can rely on is my pretty much free association instincts. I agree that this hilt seems cast, and lead would be a quite likely candidate.
Although the geometric device that comprises the neck type section between blade and hilt may be stylistically similar to such symbols in many places, it would be hard to say that these influences did not enter the presumed African tribal sphere via many avenues of trade contact. These kinds of symbolic elements are well known in art and material culture in Saharan regions to the east (see "Africa Adorned", Angela Fisher, a great work on jewelry and its symbolism in Africa). The cross of varying types is well known, and rather than religiously observed, often refers to the four cardinal directions, north, south, east and west.
This symbolic device placed at the root of the blade, as David has noted, resembles geometric symbols seen in these regions, and as the bearing sword in ceremony would be held upright with blade up, suggesting the arising or power of the blade out of the symbol, in metaphoric sense.
It should be noted that many of the falchion type swords out of the Congo/Zaire regions use this effect on thier blades with an open, bisected disc shape between the blade and hilt.
At the point in the blade above the 'nagan curve' there are what appear to be serrations on both sides of the blade, as seen on the Dahomean ceremonial swords known as 'hwi' (see "African Arms & Armour" C. Spring, and the reference to Palau Marti).
I'm not sure if this would be a trade blade, though the suggestion is well placed. The African swordsmiths seem to have long been underestimated in thier skills, as evidenced by the many elaborate shapes and styles of the blades produced there. The copper wire is also a feature often seen on the swords of Condo/Zaire regions and others in various tribal regions.
All best regards,
Jim