What Tim said about the origin of this knife is correct. These knives are used over a vast area in Africa.
The size is, in my opinion, also an indication that it's a newer piece. These knives are also called 'arm knives'. They are worn on the upper arm, just above the elbow. I think a knife as long as yours wouldn't be easy to carry in this way.
Normally, they have the size of small daggers. Both my knives measure about 29 cm (11,6 inches).
The blades of both knives are fairly thick. One even has a thick middlerib (last picture). The edges are sharpened, but as you said, they are probably more used for stabbing.
I found a picture from a book by Leni Riefenstahl, who visited remote areas of Africa to make documentaries. One of her photos a man is shown wearing a knife on his left arm (look at the man on the right). It's not a knife like the ones above, but the way of carrying it is the same.
These are Nuba of Kau, living in the south-eastern part of the Sudanese province Kordofan.
