Reminds me of the cross-section of many Dayak parangs, concave on the inside. Maybe it improves the chopping potential of the club if used to strike on a particular trajectory? After all these bladed clubs were intended to chop into the opponent rather than just bludgeon them. Cleaner bone breaks and perhaps a little cleaving into the flesh.
Shipibo and "Kayapo" clubs don't feature this - but then again that means they'll be easily used to strike on any practical swing.
