Harry:
Arif has summarised the situation. The British bit of Cameroon was an eccentric shape - I'm not sure if it tapered down to a trinational quadripoint at the Benue River. I'll read through the treaty information in Brownlie and report back. Meanwhile, I attach a scan of Brownlie's map, so that you can compare it with yours. I don't have any information about the territories you mention, since they're not listed in Brownlie. The British Order in Council for the region mentions only the Northern and Southern Provinces, the latter divided into Eastern and Western Provinces. It gives no further subdivision of Northern Provinces, but obviously these existed or the word "provinces" wouldn't have been used. Does the "split" territory you mention lie in the region of the narrow neck in the Northern Provinces?
(BTW: British Cameroon provides you tripoint enthusiasts with another pair of international tripoints, albeit asynchronous - tripoints with all boundaries having been international at some time, but not all at the same time! These are Nigeria/[Southern Provinces]/[Northern Provinces] and [Southern Provinces]/[Northern Provinces]/Cameroon. The former is marked by Boundary Post 64 on the old Anglo-German boundary survey, and the latter is on "the prominent peak which marks the Franco-British frontier" along the dividing line between British and French Cameroons.)

Grant