Nice one Lew.I have had in hands a lot of Mangbetu sickle knives when I wrote this book:
http://lulef.free.fr/html/livres.html
Your knife has a specific large appendage (often with hexagonal inverted cone) on the back and the hand made hilt (not turned) make me think of a knife from 1900 / 1920 .
The knives with ivory hilt are not all from 19th, they were made until 1930/1940 for belgian settler, but the works is different.
Luc