I guess a Krupp gun would also be ok...
Just what I was going to say.
Used by Egyptians and Mahdists (not to mention many other countries) and very easy to imagine a wealthy entrepreneurial "native" commander acquiring one. Maybe the sort of thing the Prussians would use as a "sweetener" to get a foothold in a country.
Possibly you could also use an old Napoleonic gribeauval system gun as it's just about possible one would find it's way there, or maybe a Civil War Napoleon/Parrot etc? It depends how "historical" you want your Africa to be.
I know I use all the above for my "native" forces. My Mahdists have a big old smoothbore 32 pdr ( I think) which I picked up cheap on e-bay