Thank you Kai and Willem for taking an interest! Yes this is my first mandau and one that I actually have with me (I've been displaced from home for the last 6 months). Happy to take more pictures of features of interest.
I've added a couple of what I consider similar carvings. To me these look like a hunched 6 limbed figure embracing the void of the bone cavity.
A. Is my mandau purchased from Artzi
B. Is from the 2006 post by Dajak
2006 post by Dajak
C. This is a random download from Google Image Search
I've also added a better shot of the belt and a zoom of the hair plug