Hello Fearn, here are some other pictures of the other end: there isn’t a groove but, as you said, the string is wrapped repeatedly. As you can see in the third picture there is, lightly under where now is the knot, an impression of the string. This is the sign that the other end is the part that was assembled for second.
For the leather strip I don’t think that is “the grip” and so it doesn’t slipped: in the mangbetu bows that I have seen the leather strip is in the exact position. Maybe is a grip but only for the stripping of the bow, to avoid that the bow slides in that moment. In the picture from Spring’s book (African arms and armour) is portrayed a man that stringed his bow. In the other pictures some ways to stringed bows and some African bows from the book Waffen aus Zentral Afrika
