Interesting sword Marc ! Nice blade indeed, I'm pretty sure I've seen it before, did you bought it from a French auction house by any chance ?
Surprisingly, this is not the first blade with those type of markings that I see in newer, Agadez style, mounts. One would need to look into this deeper, but there seems to be a lot of takouba bearing the snake and lion (most likely locally applied) coming from Niger. Here is 3 examples from Norman's website, but I'm pretty sure I've seen others elsewhere :
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http://takouba.org/catalog/index.php/takouba-37
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http://takouba.org/catalog/index.php/takouba-98
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http://takouba.org/catalog/index.php/takouba-101
Note how they all have the lion and snake in various configurations and styles amongst other symbols, all of them in rather recent Nigerien mounts.