Good find Sajen.
This is a link to some Ivory jewellery from Malawi attributed to the Zomba region which was settled by the Yao. The decoration is very similar to the weapon you show and identical to the designs on my knives.
linky
This picture is of Hugh Stannus Stannus one of the key donors to the BM that helps with the placement of these knives. Hugh Stannus Stannus 1877-1957 was medical officer to the King's African Rifles from 1905-1914 whilst they were based in British Central Africa (Malawi). He also published a written work on the Wa-Yao people in 1922 and was a well recognised expert in African study.
There is a more detailed description of Stannus and his work here
linky