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Though a key is a great idea, you would not be able to get much farther that general regions. Many of the types that come from specific drainages show a lot of color variability.
The best bet is to learn to tell the difference betwee the Nigerian types and Camaroonian types. A few types within these groups (like Wouri) are unique enough to key out easily, but most are not.
If you pick up a taeniatus somewhere and it is not a wild fish with a known collection place, its is enough to say 'Nigerian' or 'Camaroonian'. Aquarium strains, especially Nigerian types are so mixed these days that it is best to call them 'aquarium strains'. Many of the Camaroon types are being crossed now as well. Not a problem in my mind so long as the breeders are honest about it. Many of them locality type crosses are quite beautiful.